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diff --git a/doc/src/sgml/html/when-can-parallel-query-be-used.html b/doc/src/sgml/html/when-can-parallel-query-be-used.html new file mode 100644 index 0000000..89e20b2 --- /dev/null +++ b/doc/src/sgml/html/when-can-parallel-query-be-used.html @@ -0,0 +1,78 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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>15.2. When Can Parallel Query Be Used?</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="how-parallel-query-works.html" title="15.1. How Parallel Query Works" /><link rel="next" href="parallel-plans.html" title="15.3. Parallel Plans" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">15.2. When Can Parallel Query Be Used?</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="how-parallel-query-works.html" title="15.1. How Parallel Query Works">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 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="parallel-plans.html" title="15.3. Parallel Plans">Next</a></td></tr></table><hr /></div><div class="sect1" id="WHEN-CAN-PARALLEL-QUERY-BE-USED"><div class="titlepage"><div><div><h2 class="title" style="clear: both">15.2. When Can Parallel Query Be Used?</h2></div></div></div><p> + There are several settings that can cause the query planner not to + generate a parallel query plan under any circumstances. In order for + any parallel query plans whatsoever to be generated, the following + settings must be configured as indicated. + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS-PER-GATHER">max_parallel_workers_per_gather</a> must be set to a + value that is greater than zero. This is a special case of the more + general principle that no more workers should be used than the number + configured via <code class="varname">max_parallel_workers_per_gather</code>. + </p></li></ul></div><p> + In addition, the system must not be running in single-user mode. Since + the entire database system is running as a single process in this situation, + no background workers will be available. + </p><p> + Even when it is in general possible for parallel query plans to be + generated, the planner will not generate them for a given query + if any of the following are true: + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + The query writes any data or locks any database rows. If a query + contains a data-modifying operation either at the top level or within + a CTE, no parallel plans for that query will be generated. As an + exception, the following commands, which create a new table and populate + it, can use a parallel plan for the underlying <code class="literal">SELECT</code> + part of the query: + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem"><p><code class="command">CREATE TABLE ... AS</code></p></li><li class="listitem"><p><code class="command">SELECT INTO</code></p></li><li class="listitem"><p><code class="command">CREATE MATERIALIZED VIEW</code></p></li><li class="listitem"><p><code class="command">REFRESH MATERIALIZED VIEW</code></p></li></ul></div><p> + </p></li><li class="listitem"><p> + The query might be suspended during execution. In any situation in + which the system thinks that partial or incremental execution might + occur, no parallel plan is generated. For example, a cursor created + using <a class="link" href="sql-declare.html" title="DECLARE">DECLARE CURSOR</a> will never use + a parallel plan. Similarly, a PL/pgSQL loop of the form + <code class="literal">FOR x IN query LOOP .. END LOOP</code> will never use a + parallel plan, because the parallel query system is unable to verify + that the code in the loop is safe to execute while parallel query is + active. + </p></li><li class="listitem"><p> + The query uses any function marked <code class="literal">PARALLEL UNSAFE</code>. + Most system-defined functions are <code class="literal">PARALLEL SAFE</code>, + but user-defined functions are marked <code class="literal">PARALLEL + UNSAFE</code> by default. See the discussion of + <a class="xref" href="parallel-safety.html" title="15.4. Parallel Safety">Section 15.4</a>. + </p></li><li class="listitem"><p> + The query is running inside of another query that is already parallel. + For example, if a function called by a parallel query issues an SQL + query itself, that query will never use a parallel plan. This is a + limitation of the current implementation, but it may not be desirable + to remove this limitation, since it could result in a single query + using a very large number of processes. + </p></li></ul></div><p> + Even when parallel query plan is generated for a particular query, there + are several circumstances under which it will be impossible to execute + that plan in parallel at execution time. If this occurs, the leader + will execute the portion of the plan below the <code class="literal">Gather</code> + node entirely by itself, almost as if the <code class="literal">Gather</code> node were + not present. This will happen if any of the following conditions are met: + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + No background workers can be obtained because of the limitation that + the total number of background workers cannot exceed + <a class="xref" href="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES">max_worker_processes</a>. + </p></li><li class="listitem"><p> + No background workers can be obtained because of the limitation that + the total number of background workers launched for purposes of + parallel query cannot exceed <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS">max_parallel_workers</a>. + </p></li><li class="listitem"><p> + The client sends an Execute message with a non-zero fetch count. + See the discussion of the + <a class="link" href="protocol-flow.html#PROTOCOL-FLOW-EXT-QUERY" title="55.2.3. Extended Query">extended query protocol</a>. + Since <a class="link" href="libpq.html" title="Chapter 34. libpq — C Library">libpq</a> currently provides no way to + send such a message, this can only occur when using a client that + does not rely on libpq. If this is a frequent + occurrence, it may be a good idea to set + <a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS-PER-GATHER">max_parallel_workers_per_gather</a> to zero in + sessions where it is likely, so as to avoid generating query plans + that may be suboptimal when run serially. + </p></li></ul></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="how-parallel-query-works.html" title="15.1. How Parallel Query Works">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="parallel-plans.html" title="15.3. Parallel Plans">Next</a></td></tr><tr><td width="40%" align="left" valign="top">15.1. How Parallel Query Works </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 15.3. Parallel Plans</td></tr></table></div></body></html>
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