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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>15.4. Parallel Safety</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="parallel-plans.html" title="15.3. Parallel Plans" /><link rel="next" href="admin.html" title="Part III. Server Administration" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">15.4. Parallel Safety</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="parallel-plans.html" title="15.3. Parallel Plans">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="parallel-query.html" title="Chapter 15. Parallel Query">Up</a></td><th width="60%" align="center">Chapter 15. Parallel Query</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="admin.html" title="Part III. Server Administration">Next</a></td></tr></table><hr /></div><div class="sect1" id="PARALLEL-SAFETY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">15.4. Parallel Safety <a href="#PARALLEL-SAFETY" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="parallel-safety.html#PARALLEL-LABELING">15.4.1. Parallel Labeling for Functions and Aggregates</a></span></dt></dl></div><p>3    The planner classifies operations involved in a query as either4    <em class="firstterm">parallel safe</em>, <em class="firstterm">parallel restricted</em>,5    or <em class="firstterm">parallel unsafe</em>.  A parallel safe operation is one that6    does not conflict with the use of parallel query.  A parallel restricted7    operation is one that cannot be performed in a parallel worker, but that8    can be performed in the leader while parallel query is in use.  Therefore,9    parallel restricted operations can never occur below a <code class="literal">Gather</code>10    or <code class="literal">Gather Merge</code> node, but can occur elsewhere in a plan that11    contains such a node.  A parallel unsafe operation is one that cannot12    be performed while parallel query is in use, not even in the leader.13    When a query contains anything that is parallel unsafe, parallel query14    is completely disabled for that query.15  </p><p>16    The following operations are always parallel restricted:17  </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>18        Scans of common table expressions (CTEs).19      </p></li><li class="listitem"><p>20        Scans of temporary tables.21      </p></li><li class="listitem"><p>22        Scans of foreign tables, unless the foreign data wrapper has23        an <code class="literal">IsForeignScanParallelSafe</code> API that indicates otherwise.24      </p></li><li class="listitem"><p>25        Plan nodes to which an <code class="literal">InitPlan</code> is attached.26      </p></li><li class="listitem"><p>27        Plan nodes that reference a correlated <code class="literal">SubPlan</code>.28      </p></li></ul></div><div class="sect2" id="PARALLEL-LABELING"><div class="titlepage"><div><div><h3 class="title">15.4.1. Parallel Labeling for Functions and Aggregates <a href="#PARALLEL-LABELING" class="id_link">#</a></h3></div></div></div><p>29    The planner cannot automatically determine whether a user-defined30    function or aggregate is parallel safe, parallel restricted, or parallel31    unsafe, because this would require predicting every operation that the32    function could possibly perform.  In general, this is equivalent to the33    Halting Problem and therefore impossible.  Even for simple functions34    where it could conceivably be done, we do not try, since this would be expensive35    and error-prone.  Instead, all user-defined functions are assumed to36    be parallel unsafe unless otherwise marked.  When using37    <a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a> or38    <a class="xref" href="sql-alterfunction.html" title="ALTER FUNCTION"><span class="refentrytitle">ALTER FUNCTION</span></a>, markings can be set by specifying39    <code class="literal">PARALLEL SAFE</code>, <code class="literal">PARALLEL RESTRICTED</code>, or40    <code class="literal">PARALLEL UNSAFE</code> as appropriate.  When using41    <a class="xref" href="sql-createaggregate.html" title="CREATE AGGREGATE"><span class="refentrytitle">CREATE AGGREGATE</span></a>, the42    <code class="literal">PARALLEL</code> option can be specified with <code class="literal">SAFE</code>,43    <code class="literal">RESTRICTED</code>, or <code class="literal">UNSAFE</code> as the corresponding value.44  </p><p>45    Functions and aggregates must be marked <code class="literal">PARALLEL UNSAFE</code> if46    they write to the database, access sequences, change the transaction state47    even temporarily (e.g., a PL/pgSQL function that establishes an48    <code class="literal">EXCEPTION</code> block to catch errors), or make persistent changes to49    settings.  Similarly, functions must be marked <code class="literal">PARALLEL50    RESTRICTED</code> if they access temporary tables, client connection state,51    cursors, prepared statements, or miscellaneous backend-local state that52    the system cannot synchronize across workers. For example,53    <code class="literal">setseed</code> and <code class="literal">random</code> are parallel restricted for54    this last reason.55  </p><p>56    In general, if a function is labeled as being safe when it is restricted or57    unsafe, or if it is labeled as being restricted when it is in fact unsafe,58    it may throw errors or produce wrong answers when used in a parallel query.59    C-language functions could in theory exhibit totally undefined behavior if60    mislabeled, since there is no way for the system to protect itself against61    arbitrary C code, but in most likely cases the result will be no worse than62    for any other function. If in doubt, it is probably best to label functions63    as <code class="literal">UNSAFE</code>.64  </p><p>65    If a function executed within a parallel worker acquires locks that are66    not held by the leader, for example by querying a table not referenced in67    the query, those locks will be released at worker exit, not end of68    transaction. If you write a function that does this, and this behavior69    difference is important to you, mark such functions as70    <code class="literal">PARALLEL RESTRICTED</code>71    to ensure that they execute only in the leader.72  </p><p>73    Note that the query planner does not consider deferring the evaluation of74    parallel-restricted functions or aggregates involved in the query in75    order to obtain a superior plan.  So, for example, if a <code class="literal">WHERE</code>76    clause applied to a particular table is parallel restricted, the query77    planner will not consider performing a scan of that table in the parallel78    portion of a plan.  In some cases, it would be79    possible (and perhaps even efficient) to include the scan of that table in80    the parallel portion of the query and defer the evaluation of the81    <code class="literal">WHERE</code> clause so that it happens above the <code class="literal">Gather</code>82    node.  However, the planner does not do this.83  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="parallel-plans.html" title="15.3. Parallel Plans">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="parallel-query.html" title="Chapter 15. Parallel Query">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="admin.html" title="Part III. Server Administration">Next</a></td></tr><tr><td width="40%" align="left" valign="top">15.3. Parallel Plans </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"> Part III. Server Administration</td></tr></table></div></body></html>