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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Appendix M. Glossary</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="acronyms.html" title="Appendix L. Acronyms" /><link rel="next" href="color.html" title="Appendix N. Color Support" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Appendix M. Glossary</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="acronyms.html" title="Appendix L. Acronyms">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="appendixes.html" title="Part VIII. Appendixes">Up</a></td><th width="60%" align="center">Part VIII. Appendixes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="color.html" title="Appendix N. Color Support">Next</a></td></tr></table><hr /></div><div class="appendix" id="GLOSSARY"><div class="titlepage"><div><div><h2 class="title">Appendix M. Glossary</h2></div></div></div><p>3  This is a list of terms and their meaning in the context of4  <span class="productname">PostgreSQL</span> and relational database5  systems in general.6 </p><div class="glosslist"><dl><dt id="GLOSSARY-ACID"><span class="glossterm">ACID</span></dt><dd class="glossdef"><p>7     <a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY" title="Atomicity">Atomicity</a></em></a>,8     <a class="glossterm" href="glossary.html#GLOSSARY-CONSISTENCY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSISTENCY" title="Consistency">Consistency</a></em></a>,9     <a class="glossterm" href="glossary.html#GLOSSARY-ISOLATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ISOLATION" title="Isolation">Isolation</a></em></a>, and10     <a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY" title="Durability">Durability</a></em></a>.11     This set of properties of database transactions is intended to12     guarantee validity in concurrent operation and even in event of13     errors, power failures, etc.14    </p></dd><dt id="GLOSSARY-AGGREGATE"><span class="glossterm">Aggregate function (routine)</span></dt><dd class="glossdef"><p>15     A <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">function</a></em></a> that16     combines (<em class="firstterm">aggregates</em>) multiple input values,17     for example by counting, averaging or adding,18     yielding a single output value.19    </p><p>20     For more information, see21     <a class="xref" href="functions-aggregate.html" title="9.21. Aggregate Functions">Section 9.21</a>.22    </p><p>See Also <a class="glossseealso" href="glossary.html#GLOSSARY-WINDOW-FUNCTION">Window function (routine)</a>.</p></dd><dt><span class="glossterm">Analytic function</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WINDOW-FUNCTION">Window function (routine)</a>.</p></dd><dt id="GLOSSARY-ANALYZE"><span class="glossterm">Analyze (operation)</span></dt><dd class="glossdef"><p>23     The act of collecting statistics from data in24     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">tables</a></em></a>25     and other <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a>26     to help the <a class="glossterm" href="glossary.html#GLOSSARY-PLANNER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PLANNER" title="Query planner">query planner</a></em></a>27     to make decisions about how to execute28     <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">queries</a></em></a>.29    </p><p>30     (Don't confuse this term with the <code class="literal">ANALYZE</code> option31     to the <a class="xref" href="sql-explain.html" title="EXPLAIN"><span class="refentrytitle">EXPLAIN</span></a> command.)32    </p><p>33     For more information, see34     <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a>.35    </p></dd><dt id="GLOSSARY-ATOMIC"><span class="glossterm">Atomic</span></dt><dd class="glossdef"><p>36     In reference to a <a class="glossterm" href="glossary.html#GLOSSARY-DATUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATUM" title="Datum">datum</a></em></a>:37     the fact that its value cannot be broken down into smaller38     components.39    </p></dd><dd class="glossdef"><p>40     In reference to a41     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">database transaction</a></em></a>:42     see <a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATOMICITY" title="Atomicity">atomicity</a></em></a>.43    </p></dd><dt id="GLOSSARY-ATOMICITY"><span class="glossterm">Atomicity</span></dt><dd class="glossdef"><p>44     The property of a <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>45     that either all its operations complete as a single unit or none do.46     In addition, if a system failure occurs during the execution of a47     transaction, no partial results are visible after recovery.48     This is one of the <acronym class="acronym">ACID</acronym> properties.49    </p></dd><dt id="GLOSSARY-ATTRIBUTE"><span class="glossterm">Attribute</span></dt><dd class="glossdef"><p>50     An element with a certain name and data type found within a51     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuple</a></em></a>.52    </p></dd><dt id="GLOSSARY-AUTOVACUUM"><span class="glossterm">Autovacuum (process)</span></dt><dd class="glossdef"><p>53     A set of background processes that routinely perform54     <a class="glossterm" href="glossary.html#GLOSSARY-VACUUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VACUUM" title="Vacuum">vacuum</a></em></a>55     and <a class="glossterm" href="glossary.html#GLOSSARY-ANALYZE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ANALYZE" title="Analyze (operation)">analyze</a></em></a> operations.56     The <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>57     that coordinates the work and is always present (unless autovacuum58     is disabled) is known as the <em class="firstterm">autovacuum launcher</em>,59     and the processes that carry out the tasks are known as the60     <em class="firstterm">autovacuum workers</em>.61    </p><p>62     For more information, see63     <a class="xref" href="routine-vacuuming.html#AUTOVACUUM" title="25.1.6. The Autovacuum Daemon">Section 25.1.6</a>.64    </p></dd><dt id="GLOSSARY-AUXILIARY-PROC"><span class="glossterm">Auxiliary process</span></dt><dd class="glossdef"><p>65     A process within an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>66     that is in charge of some specific background task for the instance.67     The auxiliary processes consist of 68     69     the <a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM" title="Autovacuum (process)">autovacuum launcher</a></em></a>70     (but not the autovacuum workers),71     the <a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER" title="Background writer (process)">background writer</a></em></a>,72     the <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINTER" title="Checkpointer (process)">checkpointer</a></em></a>,73     the <a class="glossterm" href="glossary.html#GLOSSARY-LOGGER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOGGER" title="Logger (process)">logger</a></em></a>,74     the <a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS" title="Startup process">startup process</a></em></a>,75     the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-ARCHIVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-ARCHIVER" title="WAL archiver (process)">WAL archiver</a></em></a>,76     the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER" title="WAL receiver (process)">WAL receiver</a></em></a>77     (but not the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-SENDER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-SENDER" title="WAL sender (process)">WAL senders</a></em></a>),78     and the <a class="glossterm" href="glossary.html#GLOSSARY-WAL-WRITER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-WRITER" title="WAL writer (process)">WAL writer</a></em></a>.79    </p></dd><dt id="GLOSSARY-BACKEND"><span class="glossterm">Backend (process)</span></dt><dd class="glossdef"><p>80     Process of an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>81     which acts on behalf of a <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">client session</a></em></a>82     and handles its requests.83    </p><p>84     (Don't confuse this term with the similar terms85     <a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WORKER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WORKER" title="Background worker (process)">Background Worker</a></em></a> or86     <a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKGROUND-WRITER" title="Background writer (process)">Background Writer</a></em></a>).87    </p></dd><dt id="GLOSSARY-BACKGROUND-WORKER"><span class="glossterm">Background worker (process)</span></dt><dd class="glossdef"><p>88     Process within an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>,89     which runs system- or user-supplied code.90     Serves as infrastructure for several features in91     <span class="productname">PostgreSQL</span>, such as92     <a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION" title="Replication">logical replication</a></em></a>93     and <a class="glossterm" href="glossary.html#GLOSSARY-PARALLEL-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARALLEL-QUERY" title="Parallel query">parallel queries</a></em></a>.94     In addition, <a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION" title="Extension">Extensions</a></em></a> can add95     custom background worker processes.96   </p><p>97    For more information, see98    <a class="xref" href="bgworker.html" title="Chapter 48. Background Worker Processes">Chapter 48</a>.99   </p></dd><dt id="GLOSSARY-BACKGROUND-WRITER"><span class="glossterm">Background writer (process)</span></dt><dd class="glossdef"><p>100     An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>101     that writes dirty102     <a class="glossterm" href="glossary.html#GLOSSARY-DATA-PAGE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATA-PAGE" title="Data page">data pages</a></em></a> from103     <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared memory</a></em></a> to104     the file system.  It wakes up periodically, but works only for a short105     period in order to distribute its expensive <acronym class="acronym">I/O</acronym>106     activity over time to avoid generating larger107     <acronym class="acronym">I/O</acronym> peaks which could block other processes.108    </p><p>109     For more information, see110     <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-BACKGROUND-WRITER" title="20.4.5. Background Writer">Section 20.4.5</a>.111    </p></dd><dt id="GLOSSARY-BASEBACKUP"><span class="glossterm">Base Backup</span></dt><dd class="glossdef"><p>112     A binary copy of all113     <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>114     files. It is generated by the tool <a class="xref" href="app-pgbasebackup.html" title="pg_basebackup"><span class="refentrytitle"><span class="application">pg_basebackup</span></span></a>.115     In combination with WAL files it can be used as the starting point116     for recovery, log shipping, or streaming replication.117    </p></dd><dt id="GLOSSARY-BLOAT"><span class="glossterm">Bloat</span></dt><dd class="glossdef"><p>118     Space in data pages which does not contain current row versions,119     such as unused (free) space or outdated row versions.120    </p></dd><dt id="GLOSSARY-BOOTSTRAP-SUPERUSER"><span class="glossterm">Bootstrap superuser</span></dt><dd class="glossdef"><p>121     The first <a class="glossterm" href="glossary.html#GLOSSARY-USER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-USER" title="User">user</a></em></a> initialized in a122     <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>.123    </p><p>124     This user owns all system catalog tables in each database.  It is also the role125     from which all granted permissions originate.  Because of these things, this126     role may not be dropped.127    </p><p>128     This role also behaves as a normal129     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE-SUPERUSER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE-SUPERUSER" title="Database superuser">database superuser</a></em></a>.130    </p></dd><dt id="GLOSSARY-BUFFER-ACCESS-STRATEGY"><span class="glossterm">Buffer Access Strategy</span></dt><dd class="glossdef"><p>131     Some operations will access a large number of132     <a class="glossterm" href="glossary.html#GLOSSARY-DATA-PAGE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATA-PAGE" title="Data page">pages</a></em></a>.  A133     <em class="firstterm">Buffer Access Strategy</em> helps to prevent these134     operations from evicting too many pages from135     <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared buffers</a></em></a>.136    </p><p>137     A Buffer Access Strategy sets up references to a limited number of138     <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared buffers</a></em></a> and139     reuses them circularly.  When the operation requires a new page, a victim140     buffer is chosen from the buffers in the strategy ring, which may require141     flushing the page's dirty data and possibly also unflushed142     <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a> to permanent storage.143    </p><p>144     Buffer Access Strategies are used for various operations such as145     sequential scans of large tables, <code class="command">VACUUM</code>,146     <code class="command">COPY</code>, <code class="command">CREATE TABLE AS SELECT</code>,147     <code class="command">ALTER TABLE</code>, <code class="command">CREATE DATABASE</code>,148     <code class="command">CREATE INDEX</code>, and <code class="command">CLUSTER</code>.149    </p></dd><dt id="GLOSSARY-CAST"><span class="glossterm">Cast</span></dt><dd class="glossdef"><p>150     A conversion of a <a class="glossterm" href="glossary.html#GLOSSARY-DATUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATUM" title="Datum">datum</a></em></a>151     from its current data type to another data type.152    </p><p>153     For more information, see154     <a class="xref" href="sql-createcast.html" title="CREATE CAST"><span class="refentrytitle">CREATE CAST</span></a>.155    </p></dd><dt id="GLOSSARY-CATALOG"><span class="glossterm">Catalog</span></dt><dd class="glossdef"><p>156     The <acronym class="acronym">SQL</acronym> standard uses this term to157     indicate what is called a158     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> in159     <span class="productname">PostgreSQL</span>'s terminology.160    </p><p>161     (Don't confuse this term with162     <a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG" title="System catalog">system catalog</a></em></a>).163    </p><p>164     For more information, see165     <a class="xref" href="manage-ag-overview.html" title="23.1. Overview">Section 23.1</a>.166    </p></dd><dt id="GLOSSARY-CHECK-CONSTRAINT"><span class="glossterm">Check constraint</span></dt><dd class="glossdef"><p>167     A type of <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraint</a></em></a>168     defined on a <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>169     which restricts the values allowed in one or more170     <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>. The171     check constraint can make reference to any attribute of the same row in172     the relation, but cannot reference other rows of the same relation or173     other relations.174    </p><p>175     For more information, see176     <a class="xref" href="ddl-constraints.html" title="5.4. Constraints">Section 5.4</a>.177    </p></dd><dt id="GLOSSARY-CHECKPOINT"><span class="glossterm">Checkpoint</span></dt><dd class="glossdef"><p>178     A point in the <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a> sequence179     at which it is guaranteed that the heap and index data files have been180     updated with all information from181     <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared memory</a></em></a>182     modified before that checkpoint;183     a <em class="firstterm">checkpoint record</em> is written and flushed to WAL184     to mark that point.185    </p><p>186     A checkpoint is also the act of carrying out all the actions that187     are necessary to reach a checkpoint as defined above.188     This process is initiated when predefined conditions are met,189     such as a specified amount of time has passed, or a certain volume190     of records has been written; or it can be invoked by the user191     with the command <code class="command">CHECKPOINT</code>.192    </p><p>193     For more information, see194     <a class="xref" href="wal-configuration.html" title="30.5. WAL Configuration">Section 30.5</a>.195    </p></dd><dt id="GLOSSARY-CHECKPOINTER"><span class="glossterm">Checkpointer (process)</span></dt><dd class="glossdef"><p>196     An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>197     that is responsible for executing198     <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT" title="Checkpoint">checkpoints</a></em></a>.199    </p></dd><dt><span class="glossterm">Class (archaic)</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-RELATION">Relation</a>.</p></dd><dt id="GLOSSARY-CLIENT"><span class="glossterm">Client (process)</span></dt><dd class="glossdef"><p>200     Any process, possibly remote, that establishes a201     <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">session</a></em></a>202     by <a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION" title="Connection">connecting</a></em></a> to an203     <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>204     to interact with a <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.205    </p></dd><dt id="GLOSSARY-CLUSTER-OWNER"><span class="glossterm">Cluster owner</span></dt><dd class="glossdef"><p>206     The operating system user that owns the207     <a class="glossterm" href="glossary.html#GLOSSARY-DATA-DIRECTORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATA-DIRECTORY" title="Data directory">data directory</a></em></a>208     and under which the <code class="literal">postgres</code> process is run.209     It is required that this user exist prior to creating a new210     <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>.211    </p><p>212     On operating systems with a <code class="literal">root</code> user,213     said user is not allowed to be the cluster owner.214    </p></dd><dt id="GLOSSARY-COLUMN"><span class="glossterm">Column</span></dt><dd class="glossdef"><p>215     An <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attribute</a></em></a> found in216     a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> or217     <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">view</a></em></a>.218    </p></dd><dt id="GLOSSARY-COMMIT"><span class="glossterm">Commit</span></dt><dd class="glossdef"><p>219     The act of finalizing a220     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a> within221     the <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>, which222     makes it visible to other transactions and assures its223     <a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DURABILITY" title="Durability">durability</a></em></a>.224    </p><p>225     For more information, see226     <a class="xref" href="sql-commit.html" title="COMMIT"><span class="refentrytitle">COMMIT</span></a>.227    </p></dd><dt id="GLOSSARY-CONCURRENCY"><span class="glossterm">Concurrency</span></dt><dd class="glossdef"><p>228     The concept that multiple independent operations happen within the229     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> at the same time.230     In <span class="productname">PostgreSQL</span>, concurrency is controlled by231     the <a class="glossterm" href="glossary.html#GLOSSARY-MVCC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MVCC" title="Multi-version concurrency control (MVCC)">multiversion concurrency control</a></em></a>232     mechanism.233    </p></dd><dt id="GLOSSARY-CONNECTION"><span class="glossterm">Connection</span></dt><dd class="glossdef"><p>234     An established line of communication between a client process and a235     <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend</a></em></a> process,236     usually over a network, supporting a237     <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">session</a></em></a>.  This term is238     sometimes used as a synonym for session.239    </p><p>240     For more information, see241     <a class="xref" href="runtime-config-connection.html" title="20.3. Connections and Authentication">Section 20.3</a>.242    </p></dd><dt id="GLOSSARY-CONSISTENCY"><span class="glossterm">Consistency</span></dt><dd class="glossdef"><p>243     The property that the data in the244     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>245     is always in compliance with246     <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">integrity constraints</a></em></a>.247     Transactions may be allowed to violate some of the constraints248     transiently before it commits, but if such violations are not resolved249     by the time it commits, such a transaction is automatically250     <a class="glossterm" href="glossary.html#GLOSSARY-ROLLBACK"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLLBACK" title="Rollback">rolled back</a></em></a>.251     This is one of the <acronym class="acronym">ACID</acronym> properties.252    </p></dd><dt id="GLOSSARY-CONSTRAINT"><span class="glossterm">Constraint</span></dt><dd class="glossdef"><p>253     A restriction on the values of data allowed within a254     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>,255     or in attributes of a256     <a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DOMAIN" title="Domain">domain</a></em></a>.257    </p><p>258     For more information, see259     <a class="xref" href="ddl-constraints.html" title="5.4. Constraints">Section 5.4</a>.260    </p></dd><dt id="GLOSSARY-CUMULATIVE-STATISTICS"><span class="glossterm">Cumulative Statistics System</span></dt><dd class="glossdef"><p>261     A system which, if enabled, accumulates statistical information262     about the <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>'s263     activities.264    </p><p>265      For more information, see266      <a class="xref" href="monitoring-stats.html" title="28.2. The Cumulative Statistics System">Section 28.2</a>.267    </p></dd><dt><span class="glossterm">Data area</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-DATA-DIRECTORY">Data directory</a>.</p></dd><dt id="GLOSSARY-DATABASE"><span class="glossterm">Database</span></dt><dd class="glossdef"><p>268     A named collection of269     <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">local SQL objects</a></em></a>.270    </p><p>271     For more information, see272     <a class="xref" href="manage-ag-overview.html" title="23.1. Overview">Section 23.1</a>.273    </p></dd><dt id="GLOSSARY-DB-CLUSTER"><span class="glossterm">Database cluster</span></dt><dd class="glossdef"><p>274     A collection of databases and global SQL objects,275     and their common static and dynamic metadata.276     Sometimes referred to as a277     <em class="firstterm">cluster</em>.278     A database cluster is created using the279     <a class="xref" href="app-initdb.html" title="initdb"><span class="refentrytitle"><span class="application">initdb</span></span></a> program.280    </p><p>281     In <span class="productname">PostgreSQL</span>, the term282     <em class="firstterm">cluster</em> is also sometimes used to refer to an instance.283     (Don't confuse this term with the SQL command <code class="command">CLUSTER</code>.)284    </p><p>285     See also <a class="glossterm" href="glossary.html#GLOSSARY-CLUSTER-OWNER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLUSTER-OWNER" title="Cluster owner">cluster owner</a></em></a>,286     the operating-system owner of a cluster,287     and <a class="glossterm" href="glossary.html#GLOSSARY-BOOTSTRAP-SUPERUSER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BOOTSTRAP-SUPERUSER" title="Bootstrap superuser">bootstrap superuser</a></em></a>,288     the <span class="productname">PostgreSQL</span> owner of a cluster.289    </p></dd><dt><span class="glossterm">Database server</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-INSTANCE">Instance</a>.</p></dd><dt id="GLOSSARY-DATABASE-SUPERUSER"><span class="glossterm">Database superuser</span></dt><dd class="glossdef"><p>290     A role having <em class="firstterm">superuser status</em>291     (see <a class="xref" href="role-attributes.html" title="22.2. Role Attributes">Section 22.2</a>).292    </p><p>293     Frequently referred to as <em class="firstterm">superuser</em>.294    </p></dd><dt id="GLOSSARY-DATA-DIRECTORY"><span class="glossterm">Data directory</span></dt><dd class="glossdef"><p>295     The base directory on the file system of a296     <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a> that contains all297     data files and subdirectories associated with a298     <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>299     (with the exception of300     <a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE" title="Tablespace">tablespaces</a></em></a>,301     and optionally <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>).302     The environment variable <code class="literal">PGDATA</code> is commonly used to303     refer to the data directory.304    </p><p>305     A <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">cluster</a></em></a>'s storage306     space comprises the data directory plus any additional tablespaces.307    </p><p>308     For more information, see309     <a class="xref" href="storage-file-layout.html" title="73.1. Database File Layout">Section 73.1</a>.310    </p></dd><dt id="GLOSSARY-DATA-PAGE"><span class="glossterm">Data page</span></dt><dd class="glossdef"><p>311     The basic structure used to store relation data.312     All pages are of the same size.313     Data pages are typically stored on disk, each in a specific file,314     and can be read to <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared buffers</a></em></a>315     where they can be modified, becoming316     <em class="firstterm">dirty</em>.  They become clean when written317     to disk.  New pages, which initially exist in memory only, are also318     dirty until written.319    </p></dd><dt id="GLOSSARY-DATUM"><span class="glossterm">Datum</span></dt><dd class="glossdef"><p>320     The internal representation of one value of an <acronym class="acronym">SQL</acronym>321     data type.322    </p></dd><dt id="GLOSSARY-DELETE"><span class="glossterm">Delete</span></dt><dd class="glossdef"><p>323     An <acronym class="acronym">SQL</acronym> command which removes324     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a> from a given325     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>326     or <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.327    </p><p>328     For more information, see329     <a class="xref" href="sql-delete.html" title="DELETE"><span class="refentrytitle">DELETE</span></a>.330    </p></dd><dt id="GLOSSARY-DOMAIN"><span class="glossterm">Domain</span></dt><dd class="glossdef"><p>331     A user-defined data type that is based on another underlying data type.332     It acts the same as the underlying type except for possibly restricting333     the set of allowed values.334    </p><p>335     For more information, see <a class="xref" href="domains.html" title="8.18. Domain Types">Section 8.18</a>.336    </p></dd><dt id="GLOSSARY-DURABILITY"><span class="glossterm">Durability</span></dt><dd class="glossdef"><p>337     The assurance that once a338     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a> has339     been <a class="glossterm" href="glossary.html#GLOSSARY-COMMIT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COMMIT" title="Commit">committed</a></em></a>, the340     changes remain even after a system failure or crash.341     This is one of the <acronym class="acronym">ACID</acronym> properties.342    </p></dd><dt><span class="glossterm">Epoch</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-XID">Transaction ID</a>.</p></dd><dt id="GLOSSARY-EXTENSION"><span class="glossterm">Extension</span></dt><dd class="glossdef"><p>343     A software add-on package that can be installed on an344     <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a> to345     get extra features.346    </p><p>347     For more information, see348     <a class="xref" href="extend-extensions.html" title="38.17. Packaging Related Objects into an Extension">Section 38.17</a>.349    </p></dd><dt id="GLOSSARY-FILE-SEGMENT"><span class="glossterm">File segment</span></dt><dd class="glossdef"><p>350     A physical file which stores data for a given351     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.352     File segments are limited in size by a configuration value353     (typically 1 gigabyte),354     so if a relation exceeds that size, it is split into multiple segments.355    </p><p>356     For more information, see357     <a class="xref" href="storage-file-layout.html" title="73.1. Database File Layout">Section 73.1</a>.358    </p><p>359     (Don't confuse this term with the similar term360     <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL segment</a></em></a>).361    </p></dd><dt id="GLOSSARY-FOREIGN-DATA-WRAPPER"><span class="glossterm">Foreign data wrapper</span></dt><dd class="glossdef"><p>362     A means of representing data that is not contained in the local363     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> so that it appears as if were in local364     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table(s)</a></em></a>. With a foreign data wrapper it is365     possible to define a <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-SERVER" title="Foreign server">foreign server</a></em></a> and366     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign tables</a></em></a>.367    </p><p>368     For more information, see369     <a class="xref" href="sql-createforeigndatawrapper.html" title="CREATE FOREIGN DATA WRAPPER"><span class="refentrytitle">CREATE FOREIGN DATA WRAPPER</span></a>.370    </p></dd><dt id="GLOSSARY-FOREIGN-KEY"><span class="glossterm">Foreign key</span></dt><dd class="glossdef"><p>371     A type of <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraint</a></em></a>372     defined on one or more <a class="glossterm" href="glossary.html#GLOSSARY-COLUMN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COLUMN" title="Column">columns</a></em></a>373     in a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> which374     requires the value(s) in those <a class="glossterm" href="glossary.html#GLOSSARY-COLUMN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COLUMN" title="Column">columns</a></em></a> to375     identify zero or one <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">row</a></em></a>376     in another (or, infrequently, the same)377     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>.378    </p></dd><dt id="GLOSSARY-FOREIGN-SERVER"><span class="glossterm">Foreign server</span></dt><dd class="glossdef"><p>379     A named collection of380     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign tables</a></em></a> which381     all use the same382     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrapper</a></em></a>383     and have other configuration values in common.384    </p><p>385     For more information, see386     <a class="xref" href="sql-createserver.html" title="CREATE SERVER"><span class="refentrytitle">CREATE SERVER</span></a>.387    </p></dd><dt id="GLOSSARY-FOREIGN-TABLE"><span class="glossterm">Foreign table (relation)</span></dt><dd class="glossdef"><p>388     A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> which appears to have389     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a> and390     <a class="glossterm" href="glossary.html#GLOSSARY-COLUMN"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-COLUMN" title="Column">columns</a></em></a> similar to a391     regular <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>, but will forward392     requests for data through its393     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrapper</a></em></a>,394     which will return <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">result sets</a></em></a>395     structured according to the definition of the396     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign table</a></em></a>.397    </p><p>398     For more information, see399     <a class="xref" href="sql-createforeigntable.html" title="CREATE FOREIGN TABLE"><span class="refentrytitle">CREATE FOREIGN TABLE</span></a>.400    </p></dd><dt id="GLOSSARY-FORK"><span class="glossterm">Fork</span></dt><dd class="glossdef"><p>401     Each of the separate segmented file sets in which a relation is stored.402     The <em class="firstterm">main fork</em> is where the actual data resides.403     There also exist two secondary forks for metadata:404     the <a class="glossterm" href="glossary.html#GLOSSARY-FSM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FSM" title="Free space map (fork)">free space map</a></em></a>405     and the <a class="glossterm" href="glossary.html#GLOSSARY-VM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VM" title="Visibility map (fork)">visibility map</a></em></a>.406     <a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED" title="Unlogged">Unlogged relations</a></em></a>407     also have an <em class="firstterm">init fork</em>.408    </p></dd><dt id="GLOSSARY-FSM"><span class="glossterm">Free space map (fork)</span></dt><dd class="glossdef"><p>409     A storage structure that keeps metadata about each data page of a table's410     main fork.  The free space map entry for each page stores the411     amount of free space that's available for future tuples, and is structured412     to be efficiently searched for available space for a new tuple of a given413     size.414    </p><p>415     For more information, see416     <a class="xref" href="storage-fsm.html" title="73.3. Free Space Map">Section 73.3</a>.417    </p></dd><dt id="GLOSSARY-FUNCTION"><span class="glossterm">Function (routine)</span></dt><dd class="glossdef"><p>418     A type of routine that receives zero or more arguments, returns zero or more419     output values, and is constrained to run within one transaction.420     Functions are invoked as part of a query, for example via421     <code class="command">SELECT</code>.422     Certain functions can return423     <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">sets</a></em></a>; those are424     called <em class="firstterm">set-returning functions</em>.425    </p><p>426     Functions can also be used for427     <a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER" title="Trigger">triggers</a></em></a> to invoke.428    </p><p>429     For more information, see430     <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>.431    </p></dd><dt id="GLOSSARY-GRANT"><span class="glossterm">Grant</span></dt><dd class="glossdef"><p>432     An <acronym class="acronym">SQL</acronym> command that is used to allow a433     <a class="glossterm" href="glossary.html#GLOSSARY-USER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-USER" title="User">user</a></em></a> or434     <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">role</a></em></a> to access435     specific objects within the <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.436    </p><p>437     For more information, see438     <a class="xref" href="sql-grant.html" title="GRANT"><span class="refentrytitle">GRANT</span></a>.439    </p></dd><dt id="GLOSSARY-HEAP"><span class="glossterm">Heap</span></dt><dd class="glossdef"><p>440     Contains the values of <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">row</a></em></a>441     attributes (i.e., the data) for a442     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.443     The heap is realized within one or more444     <a class="glossterm" href="glossary.html#GLOSSARY-FILE-SEGMENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FILE-SEGMENT" title="File segment">file segments</a></em></a>445     in the relation's <a class="glossterm" href="glossary.html#GLOSSARY-FORK"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FORK" title="Fork">main fork</a></em></a>.446    </p></dd><dt id="GLOSSARY-HOST"><span class="glossterm">Host</span></dt><dd class="glossdef"><p>447     A computer that communicates with other computers over a network.448     This is sometimes used as a synonym for449     <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a>.450     It is also used to refer to a computer where451     <a class="glossterm" href="glossary.html#GLOSSARY-CLIENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLIENT" title="Client (process)">client processes</a></em></a> run.452    </p></dd><dt id="GLOSSARY-INDEX"><span class="glossterm">Index (relation)</span></dt><dd class="glossdef"><p>453     A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that contains454     data derived from a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>455     or <a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW" title="Materialized view (relation)">materialized view</a></em></a>.456     Its internal structure supports fast retrieval of and access to the original457     data.458    </p><p>459     For more information, see460     <a class="xref" href="sql-createindex.html" title="CREATE INDEX"><span class="refentrytitle">CREATE INDEX</span></a>.461    </p></dd><dt id="GLOSSARY-INSERT"><span class="glossterm">Insert</span></dt><dd class="glossdef"><p>462     An <acronym class="acronym">SQL</acronym> command used to add new data into a463     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>.464    </p><p>465     For more information, see466     <a class="xref" href="sql-insert.html" title="INSERT"><span class="refentrytitle">INSERT</span></a>.467    </p></dd><dt id="GLOSSARY-INSTANCE"><span class="glossterm">Instance</span></dt><dd class="glossdef"><p>468     A group of <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend</a></em></a> and469     <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary processes</a></em></a>470     that communicate using a common shared memory area.  One471     <a class="glossterm" href="glossary.html#GLOSSARY-POSTMASTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-POSTMASTER" title="Postmaster (process)">postmaster process</a></em></a>472     manages the instance; one instance manages exactly one473     <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>474     with all its databases.  Many instances can run on the same475     <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a>476     as long as their <acronym class="acronym">TCP</acronym> ports do not conflict.477    </p><p>478     The instance handles all key features of a <acronym class="acronym">DBMS</acronym>:479     read and write access to files and shared memory,480     assurance of the <acronym class="acronym">ACID</acronym> properties,481     <a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION" title="Connection">connections</a></em></a> to482     <a class="glossterm" href="glossary.html#GLOSSARY-CLIENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLIENT" title="Client (process)">client processes</a></em></a>,483     privilege verification, crash recovery, replication, etc.484    </p></dd><dt id="GLOSSARY-ISOLATION"><span class="glossterm">Isolation</span></dt><dd class="glossdef"><p>485     The property that the effects of a transaction are not visible to486     <a class="glossterm" href="glossary.html#GLOSSARY-CONCURRENCY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONCURRENCY" title="Concurrency">concurrent transactions</a></em></a>487     before it commits.488     This is one of the <acronym class="acronym">ACID</acronym> properties.489    </p><p>490     For more information, see <a class="xref" href="transaction-iso.html" title="13.2. Transaction Isolation">Section 13.2</a>.491    </p></dd><dt id="GLOSSARY-JOIN"><span class="glossterm">Join</span></dt><dd class="glossdef"><p>492     An operation and <acronym class="acronym">SQL</acronym> keyword used in493     <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">queries</a></em></a>494     for combining data from multiple495     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a>.496    </p></dd><dt id="GLOSSARY-KEY"><span class="glossterm">Key</span></dt><dd class="glossdef"><p>497     A means of identifying a <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">row</a></em></a> within a498     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> or499     other <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> by500     values contained within one or more501     <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>502     in that relation.503    </p></dd><dt id="GLOSSARY-LOCK"><span class="glossterm">Lock</span></dt><dd class="glossdef"><p>504     A mechanism that allows a process to limit or prevent simultaneous505     access to a resource.506    </p></dd><dt id="GLOSSARY-LOG-FILE"><span class="glossterm">Log file</span></dt><dd class="glossdef"><p>507     Log files contain human-readable text lines about events.508     Examples include login failures, long-running queries, etc.509    </p><p>510     For more information, see511     <a class="xref" href="logfile-maintenance.html" title="25.3. Log File Maintenance">Section 25.3</a>.512    </p></dd><dt id="GLOSSARY-LOGGED"><span class="glossterm">Logged</span></dt><dd class="glossdef"><p>513     A <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> is considered514     <a class="glossterm" href="glossary.html#GLOSSARY-LOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOGGED" title="Logged">logged</a></em></a> if changes to it are sent to the515     <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>. By default, all regular516     tables are logged. A table can be specified as517     <a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-UNLOGGED" title="Unlogged">unlogged</a></em></a> either at518     creation time or via the <code class="command">ALTER TABLE</code> command.519    </p></dd><dt id="GLOSSARY-LOGGER"><span class="glossterm">Logger (process)</span></dt><dd class="glossdef"><p>520     An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>521     which, if enabled, writes information about database events into the current522     <a class="glossterm" href="glossary.html#GLOSSARY-LOG-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOG-FILE" title="Log file">log file</a></em></a>.523     When reaching certain time- or524     volume-dependent criteria, a new log file is created.525     Also called <em class="firstterm">syslogger</em>.526    </p><p>527     For more information, see528     <a class="xref" href="runtime-config-logging.html" title="20.8. Error Reporting and Logging">Section 20.8</a>.529    </p></dd><dt id="GLOSSARY-LOG-RECORD"><span class="glossterm">Log record</span></dt><dd class="glossdef"><p>530      Archaic term for a <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL record</a></em></a>.531     </p></dd><dt id="GLOSSARY-LOG-SEQUENCE-NUMBER"><span class="glossterm">Log sequence number</span> (LSN)</dt><dd class="glossdef"><p>532     Byte offset into the <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>,533     increasing monotonically with each new <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL record</a></em></a>.534    </p><p>535     For more information, see <a class="link" href="datatype-pg-lsn.html" title="8.20. pg_lsn Type"><code class="type">pg_lsn</code></a> and <a class="xref" href="wal-internals.html" title="30.6. WAL Internals">Section 30.6</a>.536    </p></dd><dt><span class="glossterm">LSN</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-LOG-SEQUENCE-NUMBER">Log sequence number</a>.</p></dd><dt><span class="glossterm">Master (server)</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-PRIMARY-SERVER">Primary (server)</a>.</p></dd><dt id="GLOSSARY-MATERIALIZED"><span class="glossterm">Materialized</span></dt><dd class="glossdef"><p>537     The property that some information has been pre-computed and stored538     for later use, rather than computing it on-the-fly.539    </p><p>540     This term is used in541     <a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW" title="Materialized view (relation)">materialized view</a></em></a>,542     to mean that the data derived from the view's query is stored on543     disk separately from the sources of that data.544    </p><p>545     This term is also used to refer to some multi-step queries to mean that546     the data resulting from executing a given step is stored in memory547     (with the possibility of spilling to disk), so that it can be read multiple548     times by another step.549    </p></dd><dt id="GLOSSARY-MATERIALIZED-VIEW"><span class="glossterm">Materialized view (relation)</span></dt><dd class="glossdef"><p>550     A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that is551     defined by a <code class="command">SELECT</code> statement552     (just like a <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">view</a></em></a>),553     but stores data in the same way that a554     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> does. It cannot be555     modified via <code class="command">INSERT</code>, <code class="command">UPDATE</code>,556     <code class="command">DELETE</code>, or <code class="command">MERGE</code> operations.557    </p><p>558     For more information, see559     <a class="xref" href="sql-creatematerializedview.html" title="CREATE MATERIALIZED VIEW"><span class="refentrytitle">CREATE MATERIALIZED VIEW</span></a>.560    </p></dd><dt id="GLOSSARY-MERGE"><span class="glossterm">Merge</span></dt><dd class="glossdef"><p>561     An <acronym class="acronym">SQL</acronym> command used to conditionally add, modify,562     or remove <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a>563     in a given <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>,564     using data from a source565     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.566    </p><p>567     For more information, see568     <a class="xref" href="sql-merge.html" title="MERGE"><span class="refentrytitle">MERGE</span></a>.569    </p></dd><dt id="GLOSSARY-MVCC"><span class="glossterm">Multi-version concurrency control (MVCC)</span></dt><dd class="glossdef"><p>570     A mechanism designed to allow several571     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transactions</a></em></a> to be572     reading and writing the same rows without one process causing other573     processes to stall.574     In <span class="productname">PostgreSQL</span>, MVCC is implemented by575     creating copies (<em class="firstterm">versions</em>) of576     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuples</a></em></a> as they are577     modified; after transactions that can see the old versions terminate,578     those old versions need to be removed.579    </p></dd><dt id="GLOSSARY-NULL"><span class="glossterm">Null</span></dt><dd class="glossdef"><p>580     A concept of non-existence that is a central tenet of relational581     database theory. It represents the absence of a definite value.582    </p></dd><dt><span class="glossterm">Optimizer</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-PLANNER">Query planner</a>.</p></dd><dt id="GLOSSARY-PARALLEL-QUERY"><span class="glossterm">Parallel query</span></dt><dd class="glossdef"><p>583     The ability to handle parts of executing a584     <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">query</a></em></a> to take advantage585     of parallel processes on servers with multiple <acronym class="acronym">CPU</acronym>s.586    </p></dd><dt id="GLOSSARY-PARTITION"><span class="glossterm">Partition</span></dt><dd class="glossdef"><p>587     One of several disjoint (not overlapping) subsets of a larger set.588    </p></dd><dd class="glossdef"><p>589     In reference to a590     <a class="glossterm" href="glossary.html#GLOSSARY-PARTITIONED-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARTITIONED-TABLE" title="Partitioned table (relation)">partitioned table</a></em></a>:591     One of the tables that each contain part of the data of the partitioned table,592     which is said to be the <em class="firstterm">parent</em>.593     The partition is itself a table, so it can also be queried directly;594     at the same time, a partition can sometimes be a partitioned table,595     allowing hierarchies to be created.596    </p></dd><dd class="glossdef"><p>597     In reference to a <a class="glossterm" href="glossary.html#GLOSSARY-WINDOW-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WINDOW-FUNCTION" title="Window function (routine)">window function</a></em></a>598     in a <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">query</a></em></a>,599     a partition is a user-defined criterion that identifies which neighboring600     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a>601     of the <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">query's result set</a></em></a>602     can be considered by the function.603    </p></dd><dt id="GLOSSARY-PARTITIONED-TABLE"><span class="glossterm">Partitioned table (relation)</span></dt><dd class="glossdef"><p>604     A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that is605     in semantic terms the same as a <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>,606     but whose storage is distributed across several607     <a class="glossterm" href="glossary.html#GLOSSARY-PARTITION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARTITION" title="Partition">partitions</a></em></a>.608    </p></dd><dt id="GLOSSARY-POSTMASTER"><span class="glossterm">Postmaster (process)</span></dt><dd class="glossdef"><p>609      The very first process of an <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>.610      It starts and manages the611      <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary processes</a></em></a>612      and creates <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend processes</a></em></a>613      on demand.614    </p><p>615     For more information, see616     <a class="xref" href="server-start.html" title="19.3. Starting the Database Server">Section 19.3</a>.617    </p></dd><dt id="GLOSSARY-PRIMARY-KEY"><span class="glossterm">Primary key</span></dt><dd class="glossdef"><p>618     A special case of a619     <a class="glossterm" href="glossary.html#GLOSSARY-UNIQUE-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-UNIQUE-CONSTRAINT" title="Unique constraint">unique constraint</a></em></a>620     defined on a621     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> or other622     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that also623     guarantees that all of the624     <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>625     within the <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY" title="Primary key">primary key</a></em></a>626     do not have <a class="glossterm" href="glossary.html#GLOSSARY-NULL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-NULL" title="Null">null</a></em></a> values.627     As the name implies, there can be only one628     primary key per table, though it is possible to have multiple unique629     constraints that also have no null-capable attributes.630    </p></dd><dt id="GLOSSARY-PRIMARY-SERVER"><span class="glossterm">Primary (server)</span></dt><dd class="glossdef"><p>631     When two or more <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">databases</a></em></a>632     are linked via <a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION" title="Replication">replication</a></em></a>,633     the <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a>634     that is considered the authoritative source of information is called635     the <em class="firstterm">primary</em>,636     also known as a <em class="firstterm">master</em>.637    </p></dd><dt id="GLOSSARY-PROCEDURE"><span class="glossterm">Procedure (routine)</span></dt><dd class="glossdef"><p>638     A type of routine.639     Their distinctive qualities are that they do not return values,640     and that they are allowed to make transactional statements such641     as <code class="command">COMMIT</code> and <code class="command">ROLLBACK</code>.642     They are invoked via the <code class="command">CALL</code> command.643    </p><p>644     For more information, see645     <a class="xref" href="sql-createprocedure.html" title="CREATE PROCEDURE"><span class="refentrytitle">CREATE PROCEDURE</span></a>.646    </p></dd><dt id="GLOSSARY-QUERY"><span class="glossterm">Query</span></dt><dd class="glossdef"><p>647     A request sent by a client to a <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend</a></em></a>,648     usually to return results or to modify data on the database.649    </p></dd><dt id="GLOSSARY-PLANNER"><span class="glossterm">Query planner</span></dt><dd class="glossdef"><p>650     The part of <span class="productname">PostgreSQL</span> that is devoted to651     determining (<em class="firstterm">planning</em>) the most efficient way to652     execute <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">queries</a></em></a>.653     Also known as <em class="firstterm">query optimizer</em>,654     <em class="firstterm">optimizer</em>, or simply <em class="firstterm">planner</em>.655    </p></dd><dt><span class="glossterm">Record</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-TUPLE">Tuple</a>.</p></dd><dt><span class="glossterm">Recycling</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WAL-FILE">WAL file</a>.</p></dd><dt id="GLOSSARY-REFERENTIAL-INTEGRITY"><span class="glossterm">Referential integrity</span></dt><dd class="glossdef"><p>656     A means of restricting data in one <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>657     by a <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-KEY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-KEY" title="Foreign key">foreign key</a></em></a>658     so that it must have matching data in another659     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.660    </p></dd><dt id="GLOSSARY-RELATION"><span class="glossterm">Relation</span></dt><dd class="glossdef"><p>661     The generic term for all objects in a662     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>663     that have a name and a list of664     <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>665     defined in a specific order.666     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">Tables</a></em></a>,667     <a class="glossterm" href="glossary.html#GLOSSARY-SEQUENCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SEQUENCE" title="Sequence (relation)">sequences</a></em></a>,668     <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">views</a></em></a>,669     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-TABLE" title="Foreign table (relation)">foreign tables</a></em></a>,670     <a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MATERIALIZED-VIEW" title="Materialized view (relation)">materialized views</a></em></a>,671     composite types, and672     <a class="glossterm" href="glossary.html#GLOSSARY-INDEX"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INDEX" title="Index (relation)">indexes</a></em></a> are all relations.673    </p><p>674     More generically, a relation is a set of tuples; for example,675     the result of a query is also a relation.676    </p><p>677     In <span class="productname">PostgreSQL</span>,678     <em class="firstterm">Class</em> is an archaic synonym for679     <em class="firstterm">relation</em>.680    </p></dd><dt id="GLOSSARY-REPLICA"><span class="glossterm">Replica (server)</span></dt><dd class="glossdef"><p>681     A <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> that is paired682     with a <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER" title="Primary (server)">primary</a></em></a>683     database and is maintaining a copy of some or all of the primary database's684     data. The foremost reasons for doing this are to allow for greater access685     to that data, and to maintain availability of the data in the event that686     the <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER" title="Primary (server)">primary</a></em></a>687     becomes unavailable.688    </p></dd><dt id="GLOSSARY-REPLICATION"><span class="glossterm">Replication</span></dt><dd class="glossdef"><p>689     The act of reproducing data on one690     <a class="glossterm" href="glossary.html#GLOSSARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SERVER" title="Server">server</a></em></a> onto another691     server called a <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>.692     This can take the form of <em class="firstterm">physical replication</em>,693     where all file changes from one server are copied verbatim,694     or <em class="firstterm">logical replication</em> where a defined subset695     of data changes are conveyed using a higher-level representation.696    </p></dd><dt id="GLOSSARY-RESTARTPOINT"><span class="glossterm">Restartpoint</span></dt><dd class="glossdef"><p>697     A variant of a <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT" title="Checkpoint">checkpoint</a></em></a> performed on a698     <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>.699    </p><p>700     For more information, see <a class="xref" href="wal-configuration.html" title="30.5. WAL Configuration">Section 30.5</a>.701    </p></dd><dt id="GLOSSARY-RESULT-SET"><span class="glossterm">Result set</span></dt><dd class="glossdef"><p>702     A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> transmitted703     from a <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend process</a></em></a>704     to a <a class="glossterm" href="glossary.html#GLOSSARY-CLIENT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CLIENT" title="Client (process)">client</a></em></a> upon the705     completion of an <acronym class="acronym">SQL</acronym> command, usually a706     <code class="command">SELECT</code> but it can be an707     <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or708     <code class="command">DELETE</code> command if the <code class="literal">RETURNING</code>709     clause is specified.710    </p><p>711     The fact that a result set is a relation means that a query can be used712     in the definition of another query, becoming a713     <em class="firstterm">subquery</em>.714    </p></dd><dd class="glossdef"><p>715    </p></dd><dt id="GLOSSARY-REVOKE"><span class="glossterm">Revoke</span></dt><dd class="glossdef"><p>716     A command to prevent access to a named set of717     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> objects for a718     named list of <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">roles</a></em></a>.719    </p><p>720     For more information, see721     <a class="xref" href="sql-revoke.html" title="REVOKE"><span class="refentrytitle">REVOKE</span></a>.722    </p></dd><dt id="GLOSSARY-ROLE"><span class="glossterm">Role</span></dt><dd class="glossdef"><p>723     A collection of access privileges to the724     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">instance</a></em></a>.725     Roles are themselves a privilege that can be granted to other roles.726     This is often done for convenience or to ensure completeness727     when multiple <a class="glossterm" href="glossary.html#GLOSSARY-USER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-USER" title="User">users</a></em></a> need728     the same privileges.729    </p><p>730     For more information, see731     <a class="xref" href="sql-createrole.html" title="CREATE ROLE"><span class="refentrytitle">CREATE ROLE</span></a>.732    </p></dd><dt id="GLOSSARY-ROLLBACK"><span class="glossterm">Rollback</span></dt><dd class="glossdef"><p>733     A command to undo all of the operations performed since the beginning734     of a <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>.735    </p><p>736     For more information, see737     <a class="xref" href="sql-rollback.html" title="ROLLBACK"><span class="refentrytitle">ROLLBACK</span></a>.738    </p></dd><dt id="GLOSSARY-ROUTINE"><span class="glossterm">Routine</span></dt><dd class="glossdef"><p>739     A defined set of instructions stored in the database system740     that can be invoked for execution.741     A routine can be written in a variety of programming742     languages.  Routines can be743     <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">functions</a></em></a>744     (including set-returning functions and745     <a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRIGGER" title="Trigger">trigger functions</a></em></a>),746     <a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE" title="Aggregate function (routine)">aggregate functions</a></em></a>,747     and <a class="glossterm" href="glossary.html#GLOSSARY-PROCEDURE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PROCEDURE" title="Procedure (routine)">procedures</a></em></a>.748    </p><p>749     Many routines are already defined within <span class="productname">PostgreSQL</span>750     itself, but user-defined ones can also be added.751    </p></dd><dt><span class="glossterm">Row</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-TUPLE">Tuple</a>.</p></dd><dt id="GLOSSARY-SAVEPOINT"><span class="glossterm">Savepoint</span></dt><dd class="glossdef"><p>752     A special mark in the sequence of steps in a753     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>.754     Data modifications after this point in time may be reverted755     to the time of the savepoint.756    </p><p>757     For more information, see758     <a class="xref" href="sql-savepoint.html" title="SAVEPOINT"><span class="refentrytitle">SAVEPOINT</span></a>.759    </p></dd><dt id="GLOSSARY-SCHEMA"><span class="glossterm">Schema</span></dt><dd class="glossdef"><p>760     A schema is a namespace for761     <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">SQL objects</a></em></a>,762     which all reside in the same763     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.764     Each SQL object must reside in exactly one schema.765    </p><p>766     All system-defined SQL objects reside in schema <code class="literal">pg_catalog</code>.767    </p></dd><dd class="glossdef"><p>768     More generically, the term <em class="firstterm">schema</em> is used to mean769     all data descriptions (<a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a> definitions,770     <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraints</a></em></a>, comments, etc.)771     for a given <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> or772     subset thereof.773    </p><p>774     For more information, see775     <a class="xref" href="ddl-schemas.html" title="5.9. Schemas">Section 5.9</a>.776    </p></dd><dt><span class="glossterm">Segment</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-FILE-SEGMENT">File segment</a>.</p></dd><dt id="GLOSSARY-SELECT"><span class="glossterm">Select</span></dt><dd class="glossdef"><p>777     The <acronym class="acronym">SQL</acronym> command used to request data from a778     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>.779     Normally, <code class="command">SELECT</code> commands are not expected to modify the780     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> in any way,781     but it is possible that782     <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">functions</a></em></a> invoked within783     the query could have side effects that do modify data.784    </p><p>785     For more information, see786     <a class="xref" href="sql-select.html" title="SELECT"><span class="refentrytitle">SELECT</span></a>.787    </p></dd><dt id="GLOSSARY-SEQUENCE"><span class="glossterm">Sequence (relation)</span></dt><dd class="glossdef"><p>788     A type of relation that is used to generate values.789     Typically the generated values are sequential non-repeating numbers.790     They are commonly used to generate surrogate791     <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-KEY" title="Primary key">primary key</a></em></a>792     values.793    </p></dd><dt id="GLOSSARY-SERVER"><span class="glossterm">Server</span></dt><dd class="glossdef"><p>794     A computer on which <span class="productname">PostgreSQL</span>795     <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instances</a></em></a> run.796     The term <em class="firstterm">server</em> denotes real hardware, a797     container, or a <em class="firstterm">virtual machine</em>.798    </p><p>799     This term is sometimes used to refer to an instance or to a host.800    </p></dd><dt id="GLOSSARY-SESSION"><span class="glossterm">Session</span></dt><dd class="glossdef"><p>801     A state that allows a client and a backend to interact,802     communicating over a <a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONNECTION" title="Connection">connection</a></em></a>.803    </p></dd><dt id="GLOSSARY-SHARED-MEMORY"><span class="glossterm">Shared memory</span></dt><dd class="glossdef"><p>804     <acronym class="acronym">RAM</acronym> which is used by the processes common to an805     <a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-INSTANCE" title="Instance">instance</a></em></a>.806     It mirrors parts of <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a>807     files, provides a transient area for808     <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL records</a></em></a>,809     and stores additional common information.810     Note that shared memory belongs to the complete instance, not to a single811     database.812    </p><p>813     The largest part of shared memory is known as <em class="firstterm">shared buffers</em>814     and is used to mirror part of data files, organized into pages.815     When a page is modified, it is called a dirty page until it is816     written back to the file system.817    </p><p>818     For more information, see819     <a class="xref" href="runtime-config-resource.html#RUNTIME-CONFIG-RESOURCE-MEMORY" title="20.4.1. Memory">Section 20.4.1</a>.820    </p></dd><dt id="GLOSSARY-SQL-OBJECT"><span class="glossterm">SQL object</span></dt><dd class="glossdef"><p>821      Any object that can be created with a <code class="command">CREATE</code>822      command.  Most objects are specific to one database, and are commonly823      known as <em class="firstterm">local objects</em>.824     </p><p>825      Most local objects reside in a specific826      <a class="glossterm" href="glossary.html#GLOSSARY-SCHEMA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SCHEMA" title="Schema">schema</a></em></a> in their827      containing database, such as828      <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a> (all types),829      <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">routines</a></em></a> (all types),830      data types, etc.831      The names of such objects of the same type in the same schema832      are enforced to be unique.833     </p><p>834      There also exist local objects that do not reside in schemas; some examples are835      <a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-EXTENSION" title="Extension">extensions</a></em></a>,836      <a class="glossterm" href="glossary.html#GLOSSARY-CAST"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CAST" title="Cast">data type casts</a></em></a>, and837      <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrappers</a></em></a>.838      The names of such objects of the same type are enforced to be unique839      within the database.840     </p><p>841      Other object types, such as842      <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">roles</a></em></a>,843      <a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLESPACE" title="Tablespace">tablespaces</a></em></a>,844      replication origins, subscriptions for logical replication, and845      databases themselves are not local SQL objects since they exist846      entirely outside of any specific database;847      they are called <em class="firstterm">global objects</em>.848      The names of such objects are enforced to be unique within the whole849      database cluster.850     </p><p>851      For more information, see852      <a class="xref" href="manage-ag-overview.html" title="23.1. Overview">Section 23.1</a>.853    </p></dd><dt id="GLOSSARY-SQL-STANDARD"><span class="glossterm">SQL standard</span></dt><dd class="glossdef"><p>854     A series of documents that define the <acronym class="acronym">SQL</acronym> language.855    </p></dd><dt><span class="glossterm">Standby (server)</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-REPLICA">Replica (server)</a>.</p></dd><dt id="GLOSSARY-STARTUP-PROCESS"><span class="glossterm">Startup process</span></dt><dd class="glossdef"><p>856     An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>857     that replays WAL during crash recovery and in a858     <a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICATION" title="Replication">physical replica</a></em></a>.859    </p><p>860     (The name is historical: the startup process was named before861     replication was implemented; the name refers to its task as it862     relates to the server startup following a crash.)863    </p></dd><dt id="GLOSSARY-SUPERUSER"><span class="glossterm">Superuser</span></dt><dd class="glossdef"><p>864     As used in this documentation, it is a synonym for865     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE-SUPERUSER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE-SUPERUSER" title="Database superuser">database superuser</a></em></a>.866    </p></dd><dt id="GLOSSARY-SYSTEM-CATALOG"><span class="glossterm">System catalog</span></dt><dd class="glossdef"><p>867     A collection of <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">tables</a></em></a>868     which describe the structure of all869     <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">SQL objects</a></em></a>870     of the instance.871     The system catalog resides in the schema <code class="literal">pg_catalog</code>.872     These tables contain data in internal representation and are873     not typically considered useful for user examination;874     a number of user-friendlier <a class="glossterm" href="glossary.html#GLOSSARY-VIEW"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-VIEW" title="View">views</a></em></a>,875     also in schema <code class="literal">pg_catalog</code>, offer more convenient access to876     some of that information, while additional tables and views877     exist in schema <code class="literal">information_schema</code>878     (see <a class="xref" href="information-schema.html" title="Chapter 37. The Information Schema">Chapter 37</a>) that expose some879     of the same and additional information as mandated by the880     <a class="glossterm" href="glossary.html#GLOSSARY-SQL-STANDARD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-STANDARD" title="SQL standard">SQL standard</a></em></a>.881    </p><p>882      For more information, see883      <a class="xref" href="ddl-schemas.html" title="5.9. Schemas">Section 5.9</a>.884    </p></dd><dt id="GLOSSARY-TABLE"><span class="glossterm">Table</span></dt><dd class="glossdef"><p>885     A collection of <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuples</a></em></a> having886     a common data structure (the same number of887     <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>, in the same888     order, having the same name and type per position).889     A table is the most common form of890     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> in891     <span class="productname">PostgreSQL</span>.892    </p><p>893     For more information, see894     <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>.895    </p></dd><dt id="GLOSSARY-TABLESPACE"><span class="glossterm">Tablespace</span></dt><dd class="glossdef"><p>896     A named location on the server file system.897     All <a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SQL-OBJECT" title="SQL object">SQL objects</a></em></a>898     which require storage beyond their definition in the899     <a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SYSTEM-CATALOG" title="System catalog">system catalog</a></em></a>900     must belong to a single tablespace.901     Initially, a database cluster contains a single usable tablespace which is902     used as the default for all SQL objects, called <code class="literal">pg_default</code>.903    </p><p>904     For more information, see905     <a class="xref" href="manage-ag-tablespaces.html" title="23.6. Tablespaces">Section 23.6</a>.906    </p></dd><dt id="GLOSSARY-TEMPORARY-TABLE"><span class="glossterm">Temporary table</span></dt><dd class="glossdef"><p>907     <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">Tables</a></em></a> that exist either908     for the lifetime of a909     <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">session</a></em></a> or a910     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a>, as911     specified at the time of creation.912     The data in them is not visible to other sessions, and is not913     <a class="glossterm" href="glossary.html#GLOSSARY-LOGGED"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-LOGGED" title="Logged">logged</a></em></a>.914     Temporary tables are often used to store intermediate data for a915     multi-step operation.916    </p><p>917     For more information, see918     <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a>.919    </p></dd><dt id="GLOSSARY-TOAST"><span class="glossterm">TOAST</span></dt><dd class="glossdef"><p>920     A mechanism by which large attributes of table rows are split and921     stored in a secondary table, called the <em class="firstterm">TOAST table</em>.922     Each relation with large attributes has its own TOAST table.923    </p><p>924     For more information, see925     <a class="xref" href="storage-toast.html" title="73.2. TOAST">Section 73.2</a>.926    </p></dd><dt id="GLOSSARY-TRANSACTION"><span class="glossterm">Transaction</span></dt><dd class="glossdef"><p>927     A combination of commands that must act as a single928     <a class="glossterm" href="glossary.html#GLOSSARY-ATOMIC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATOMIC" title="Atomic">atomic</a></em></a> command: they all929     succeed or all fail as a single unit, and their effects are not visible to930     other <a class="glossterm" href="glossary.html#GLOSSARY-SESSION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SESSION" title="Session">sessions</a></em></a> until931     the transaction is complete, and possibly even later, depending on the932     isolation level.933    </p><p>934     For more information, see935     <a class="xref" href="transaction-iso.html" title="13.2. Transaction Isolation">Section 13.2</a>.936    </p></dd><dt id="GLOSSARY-XID"><span class="glossterm">Transaction ID</span></dt><dd class="glossdef"><p>937     The numerical, unique, sequentially-assigned identifier that each938     transaction receives when it first causes a database modification.939     Frequently abbreviated as <em class="firstterm">xid</em>.940     When stored on disk, xids are only 32-bits wide, so only941     approximately four billion write transaction IDs can be generated;942     to permit the system to run for longer than that,943     <em class="firstterm">epochs</em> are used, also 32 bits wide.944     When the counter reaches the maximum xid value, it starts over at945     <code class="literal">3</code> (values under that are reserved) and the946     epoch value is incremented by one.947     In some contexts, the epoch and xid values are948     considered together as a single 64-bit value;  see <a class="xref" href="transaction-id.html" title="74.1. Transactions and Identifiers">Section 74.1</a> for more details.949    </p><p>950     For more information, see951     <a class="xref" href="datatype-oid.html" title="8.19. Object Identifier Types">Section 8.19</a>.952    </p></dd><dt id="GLOSSARY-TPS"><span class="glossterm">Transactions per second (TPS)</span></dt><dd class="glossdef"><p>953     Average number of transactions that are executed per second,954     totaled across all sessions active for a measured run.955     This is used as a measure of the performance characteristics of956     an instance.957    </p></dd><dt id="GLOSSARY-TRIGGER"><span class="glossterm">Trigger</span></dt><dd class="glossdef"><p>958     A <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">function</a></em></a> which can959     be defined to execute whenever a certain operation (<code class="command">INSERT</code>,960     <code class="command">UPDATE</code>, <code class="command">DELETE</code>,961     <code class="command">TRUNCATE</code>) is applied to a962     <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>.963     A trigger executes within the same964     <a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TRANSACTION" title="Transaction">transaction</a></em></a> as the965     statement which invoked it, and if the function fails, then the invoking966     statement also fails.967    </p><p>968     For more information, see969     <a class="xref" href="sql-createtrigger.html" title="CREATE TRIGGER"><span class="refentrytitle">CREATE TRIGGER</span></a>.970    </p></dd><dt id="GLOSSARY-TUPLE"><span class="glossterm">Tuple</span></dt><dd class="glossdef"><p>971     A collection of <a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ATTRIBUTE" title="Attribute">attributes</a></em></a>972     in a fixed order.973     That order may be defined by the <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>974     (or other <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>)975     where the tuple is contained, in which case the tuple is often called a976     <em class="firstterm">row</em>.  It may also be defined by the structure of a977     result set, in which case it is sometimes called a <em class="firstterm">record</em>.978    </p></dd><dt id="GLOSSARY-UNIQUE-CONSTRAINT"><span class="glossterm">Unique constraint</span></dt><dd class="glossdef"><p>979     A type of <a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CONSTRAINT" title="Constraint">constraint</a></em></a>980     defined on a <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a>981     which restricts the values allowed in one or a combination of columns982     so that each value or combination of values can only appear once in the983     relation — that is, no other row in the relation contains values984     that are equal to those.985    </p><p>986     Because <a class="glossterm" href="glossary.html#GLOSSARY-NULL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-NULL" title="Null">null values</a></em></a> are987     not considered equal to each other, multiple rows with null values are988     allowed to exist without violating the unique constraint.989    </p></dd><dt id="GLOSSARY-UNLOGGED"><span class="glossterm">Unlogged</span></dt><dd class="glossdef"><p>990     The property of certain <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relations</a></em></a>991     that the changes to them are not reflected in the992     <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>.993     This disables replication and crash recovery for these relations.994    </p><p>995     The primary use of unlogged tables is for storing996     transient work data that must be shared across processes.997    </p><p>998     <a class="glossterm" href="glossary.html#GLOSSARY-TEMPORARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TEMPORARY-TABLE" title="Temporary table">Temporary tables</a></em></a>999     are always unlogged.1000    </p></dd><dt id="GLOSSARY-UPDATE"><span class="glossterm">Update</span></dt><dd class="glossdef"><p>1001     An <acronym class="acronym">SQL</acronym> command used to modify1002     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a>1003     that may already exist in a specified <a class="glossterm" href="glossary.html#GLOSSARY-TABLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TABLE" title="Table">table</a></em></a>.1004     It cannot create or remove rows.1005    </p><p>1006     For more information, see1007     <a class="xref" href="sql-update.html" title="UPDATE"><span class="refentrytitle">UPDATE</span></a>.1008    </p></dd><dt id="GLOSSARY-USER"><span class="glossterm">User</span></dt><dd class="glossdef"><p>1009     A <a class="glossterm" href="glossary.html#GLOSSARY-ROLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-ROLE" title="Role">role</a></em></a> that has the1010     <em class="firstterm">login privilege</em>1011     (see <a class="xref" href="role-attributes.html" title="22.2. Role Attributes">Section 22.2</a>).1012    </p></dd><dt id="GLOSSARY-USER-MAPPING"><span class="glossterm">User mapping</span></dt><dd class="glossdef"><p>1013     The translation of login credentials in the local1014     <a class="glossterm" href="glossary.html#GLOSSARY-DATABASE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DATABASE" title="Database">database</a></em></a> to credentials1015     in a remote data system defined by a1016     <a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FOREIGN-DATA-WRAPPER" title="Foreign data wrapper">foreign data wrapper</a></em></a>.1017    </p><p>1018     For more information, see1019     <a class="xref" href="sql-createusermapping.html" title="CREATE USER MAPPING"><span class="refentrytitle">CREATE USER MAPPING</span></a>.1020    </p></dd><dt id="GLOSSARY-VACUUM"><span class="glossterm">Vacuum</span></dt><dd class="glossdef"><p>1021     The process of removing outdated1022     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">tuple versions</a></em></a>1023     from tables or materialized views, and other closely related1024     processing required by <span class="productname">PostgreSQL</span>'s1025     implementation of <a class="glossterm" href="glossary.html#GLOSSARY-MVCC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-MVCC" title="Multi-version concurrency control (MVCC)">MVCC</a></em></a>.1026     This can be initiated through the use of1027     the <code class="command">VACUUM</code> command, but can also be handled automatically1028     via <a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUTOVACUUM" title="Autovacuum (process)">autovacuum</a></em></a> processes.1029    </p><p>1030     For more information, see1031     <a class="xref" href="routine-vacuuming.html" title="25.1. Routine Vacuuming">Section 25.1</a> .1032    </p></dd><dt id="GLOSSARY-VIEW"><span class="glossterm">View</span></dt><dd class="glossdef"><p>1033     A <a class="glossterm" href="glossary.html#GLOSSARY-RELATION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RELATION" title="Relation">relation</a></em></a> that is defined by a1034     <code class="command">SELECT</code> statement, but has no storage of its own.1035     Any time a query references a view, the definition of the view is1036     substituted into the query as if the user had typed it as a subquery1037     instead of the name of the view.1038    </p><p>1039     For more information, see1040     <a class="xref" href="sql-createview.html" title="CREATE VIEW"><span class="refentrytitle">CREATE VIEW</span></a>.1041    </p></dd><dt id="GLOSSARY-VM"><span class="glossterm">Visibility map (fork)</span></dt><dd class="glossdef"><p>1042     A storage structure that keeps metadata about each data page1043     of a table's main fork.  The visibility map entry for1044     each page stores two bits: the first one1045     (<code class="literal">all-visible</code>) indicates that all tuples1046     in the page are visible to all transactions.  The second one1047     (<code class="literal">all-frozen</code>) indicates that all tuples1048     in the page are marked frozen.1049    </p></dd><dt><span class="glossterm">WAL</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WAL">Write-ahead log</a>.</p></dd><dt id="GLOSSARY-WAL-ARCHIVER"><span class="glossterm">WAL archiver (process)</span></dt><dd class="glossdef"><p>1050     An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>1051     which, if enabled, saves copies of1052     <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL files</a></em></a>1053     for the purpose of creating backups or keeping1054     <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replicas</a></em></a> current.1055    </p><p>1056     For more information, see1057     <a class="xref" href="continuous-archiving.html" title="26.3. Continuous Archiving and Point-in-Time Recovery (PITR)">Section 26.3</a>.1058    </p></dd><dt id="GLOSSARY-WAL-FILE"><span class="glossterm">WAL file</span></dt><dd class="glossdef"><p>1059     Also known as <em class="firstterm">WAL segment</em> or1060     <em class="firstterm">WAL segment file</em>.1061     Each of the sequentially-numbered files that provide storage space for1062     <a class="glossterm" href="glossary.html#GLOSSARY-WAL"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL" title="Write-ahead log">WAL</a></em></a>.1063     The files are all of the same predefined size1064     and are written in sequential order, interspersing changes1065     as they occur in multiple simultaneous sessions.1066     If the system crashes, the files are read in order, and each of the1067     changes is replayed to restore the system to the state it was in1068     before the crash.1069    </p><p>1070     Each WAL file can be released after a1071     <a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-CHECKPOINT" title="Checkpoint">checkpoint</a></em></a>1072     writes all the changes in it to the corresponding data files.1073     Releasing the file can be done either by deleting it, or by changing its1074     name so that it will be used in the future, which is called1075     <em class="firstterm">recycling</em>.1076    </p><p>1077     For more information, see1078     <a class="xref" href="wal-internals.html" title="30.6. WAL Internals">Section 30.6</a>.1079    </p></dd><dt id="GLOSSARY-WAL-RECORD"><span class="glossterm">WAL record</span></dt><dd class="glossdef"><p>1080     A low-level description of an individual data change.1081     It contains sufficient information for the data change to be1082     re-executed (<em class="firstterm">replayed</em>) in case a system failure1083     causes the change to be lost.1084     WAL records use a non-printable binary format.1085    </p><p>1086     For more information, see1087     <a class="xref" href="wal-internals.html" title="30.6. WAL Internals">Section 30.6</a>.1088    </p></dd><dt id="GLOSSARY-WAL-RECEIVER"><span class="glossterm">WAL receiver (process)</span></dt><dd class="glossdef"><p>1089     An <a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AUXILIARY-PROC" title="Auxiliary process">auxiliary process</a></em></a>1090     that runs on a <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>1091     to receive WAL from the1092     <a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PRIMARY-SERVER" title="Primary (server)">primary server</a></em></a>1093     for replay by the1094     <a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-STARTUP-PROCESS" title="Startup process">startup process</a></em></a>.1095    </p><p>1096     For more information, see1097     <a class="xref" href="warm-standby.html" title="27.2. Log-Shipping Standby Servers">Section 27.2</a>.1098    </p></dd><dt><span class="glossterm">WAL segment</span></dt><dd><p>See <a class="glosssee" href="glossary.html#GLOSSARY-WAL-FILE">WAL file</a>.</p></dd><dt id="GLOSSARY-WAL-SENDER"><span class="glossterm">WAL sender (process)</span></dt><dd class="glossdef"><p>1099     A special <a class="glossterm" href="glossary.html#GLOSSARY-BACKEND"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-BACKEND" title="Backend (process)">backend process</a></em></a>1100     that streams WAL over a network.  The receiving end can be a1101     <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECEIVER" title="WAL receiver (process)">WAL receiver</a></em></a>1102     in a <a class="glossterm" href="glossary.html#GLOSSARY-REPLICA"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-REPLICA" title="Replica (server)">replica</a></em></a>,1103     <a class="xref" href="app-pgreceivewal.html" title="pg_receivewal"><span class="refentrytitle"><span class="application">pg_receivewal</span></span></a>, or any other client program1104     that speaks the replication protocol.1105    </p></dd><dt id="GLOSSARY-WAL-WRITER"><span class="glossterm">WAL writer (process)</span></dt><dd class="glossdef"><p>1106     A process that writes <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL records</a></em></a>1107     from <a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-SHARED-MEMORY" title="Shared memory">shared memory</a></em></a> to1108     <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL files</a></em></a>.1109    </p><p>1110     For more information, see1111     <a class="xref" href="runtime-config-wal.html" title="20.5. Write Ahead Log">Section 20.5</a>.1112    </p></dd><dt id="GLOSSARY-WINDOW-FUNCTION"><span class="glossterm">Window function (routine)</span></dt><dd class="glossdef"><p>1113     A type of <a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-FUNCTION" title="Function (routine)">function</a></em></a>1114     used in a <a class="glossterm" href="glossary.html#GLOSSARY-QUERY"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-QUERY" title="Query">query</a></em></a>1115     that applies to a <a class="glossterm" href="glossary.html#GLOSSARY-PARTITION"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-PARTITION" title="Partition">partition</a></em></a>1116     of the query's <a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-RESULT-SET" title="Result set">result set</a></em></a>;1117     the function's result is based on values found in1118     <a class="glossterm" href="glossary.html#GLOSSARY-TUPLE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-TUPLE" title="Tuple">rows</a></em></a> of the same partition or frame.1119    </p><p>1120     All <a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-AGGREGATE" title="Aggregate function (routine)">aggregate functions</a></em></a>1121     can be used as window functions, but window functions can also be1122     used to, for example, give ranks to each of the rows in the partition.1123     Also known as <em class="firstterm">analytic functions</em>.1124    </p><p>1125     For more information, see1126     <a class="xref" href="tutorial-window.html" title="3.5. Window Functions">Section 3.5</a>.1127    </p></dd><dt id="GLOSSARY-WAL"><span class="glossterm">Write-ahead log</span></dt><dd class="glossdef"><p>1128     The journal that keeps track of the changes in the1129     <a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-DB-CLUSTER" title="Database cluster">database cluster</a></em></a>1130     as user- and system-invoked operations take place.1131     It comprises many individual1132     <a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-RECORD" title="WAL record">WAL records</a></em></a> written1133     sequentially to <a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE"><em class="glossterm"><a class="glossterm" href="glossary.html#GLOSSARY-WAL-FILE" title="WAL file">WAL files</a></em></a>.1134    </p></dd></dl></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="acronyms.html" title="Appendix L. Acronyms">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendixes.html" title="Part VIII. Appendixes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="color.html" title="Appendix N. Color Support">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Appendix L. Acronyms </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 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codekingpro/portable-devtools · Team Ai