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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>52.6. Executor</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="planner-optimizer.html" title="52.5. Planner/Optimizer" /><link rel="next" href="catalogs.html" title="Chapter 53. System Catalogs" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">52.6. Executor</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="planner-optimizer.html" title="52.5. Planner/Optimizer">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Up</a></td><th width="60%" align="center">Chapter 52. Overview of PostgreSQL Internals</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="catalogs.html" title="Chapter 53. System Catalogs">Next</a></td></tr></table><hr /></div><div class="sect1" id="EXECUTOR"><div class="titlepage"><div><div><h2 class="title" style="clear: both">52.6. Executor <a href="#EXECUTOR" class="id_link">#</a></h2></div></div></div><p>3    The <em class="firstterm">executor</em> takes the plan created by the4    planner/optimizer and recursively processes it to extract the required set5    of rows.  This is essentially a demand-pull pipeline mechanism.6    Each time a plan node is called, it must deliver one more row, or7    report that it is done delivering rows.8   </p><p>9    To provide a concrete example, assume that the top10    node is a <code class="literal">MergeJoin</code> node.11    Before any merge can be done two rows have to be fetched (one from12    each subplan). So the executor recursively calls itself to13    process the subplans (it starts with the subplan attached to14    <code class="literal">lefttree</code>). The new top node (the top node of the left15    subplan) is, let's say, a16    <code class="literal">Sort</code> node and again recursion is needed to obtain17    an input row.  The child node of the <code class="literal">Sort</code> might18    be a <code class="literal">SeqScan</code> node, representing actual reading of a table.19    Execution of this node causes the executor to fetch a row from the20    table and return it up to the calling node.  The <code class="literal">Sort</code>21    node will repeatedly call its child to obtain all the rows to be sorted.22    When the input is exhausted (as indicated by the child node returning23    a NULL instead of a row), the <code class="literal">Sort</code> code performs24    the sort, and finally is able to return its first output row, namely25    the first one in sorted order.  It keeps the remaining rows stored so26    that it can deliver them in sorted order in response to later demands.27   </p><p>28    The <code class="literal">MergeJoin</code> node similarly demands the first row29    from its right subplan.  Then it compares the two rows to see if they30    can be joined; if so, it returns a join row to its caller.  On the next31    call, or immediately if it cannot join the current pair of inputs,32    it advances to the next row of one table33    or the other (depending on how the comparison came out), and again34    checks for a match.  Eventually, one subplan or the other is exhausted,35    and the <code class="literal">MergeJoin</code> node returns NULL to indicate that36    no more join rows can be formed.37   </p><p>38    Complex queries can involve many levels of plan nodes, but the general39    approach is the same: each node computes and returns its next output40    row each time it is called.  Each node is also responsible for applying41    any selection or projection expressions that were assigned to it by42    the planner.43   </p><p>44    The executor mechanism is used to evaluate all five basic SQL query45    types: <code class="command">SELECT</code>, <code class="command">INSERT</code>,46    <code class="command">UPDATE</code>, <code class="command">DELETE</code>, and47    <code class="command">MERGE</code>.48    For <code class="command">SELECT</code>, the top-level executor code49    only needs to send each row returned by the query plan tree50    off to the client.  <code class="command">INSERT ... SELECT</code>,51    <code class="command">UPDATE</code>, <code class="command">DELETE</code>, and52    <code class="command">MERGE</code>53    are effectively <code class="command">SELECT</code>s under a special54    top-level plan node called <code class="literal">ModifyTable</code>.55   </p><p>56    <code class="command">INSERT ... SELECT</code> feeds the rows up57    to <code class="literal">ModifyTable</code> for insertion.  For58    <code class="command">UPDATE</code>, the planner arranges that each59    computed row includes all the updated column values, plus the60    <em class="firstterm">TID</em> (tuple ID, or row ID) of the original61    target row; this data is fed up to the <code class="literal">ModifyTable</code>62    node, which uses the information to create a new updated row and63    mark the old row deleted.  For <code class="command">DELETE</code>, the only64    column that is actually returned by the plan is the TID, and the65    <code class="literal">ModifyTable</code> node simply uses the TID to visit each66    target row and mark it deleted.  For <code class="command">MERGE</code>, the67    planner joins the source and target relations, and includes all68    column values required by any of the <code class="literal">WHEN</code> clauses,69    plus the TID of the target row; this data is fed up to the70    <code class="literal">ModifyTable</code> node, which uses the information to71    work out which <code class="literal">WHEN</code> clause to execute, and then72    inserts, updates or deletes the target row, as required.73   </p><p>74    A simple <code class="command">INSERT ... VALUES</code> command creates a75    trivial plan tree consisting of a single <code class="literal">Result</code>76    node, which computes just one result row, feeding that up77    to <code class="literal">ModifyTable</code> to perform the insertion.78   </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="planner-optimizer.html" title="52.5. Planner/Optimizer">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="overview.html" title="Chapter 52. Overview of PostgreSQL Internals">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="catalogs.html" title="Chapter 53. System Catalogs">Next</a></td></tr><tr><td width="40%" align="left" valign="top">52.5. Planner/Optimizer </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 53. System Catalogs</td></tr></table></div></body></html>