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- <!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 V1.79.1" /><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><div xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 12.4 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></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 which 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 which 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 in 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 commands <code class="literal">CREATE TABLE ... AS</code>, <code class="literal">SELECT
- INTO</code>, and <code class="literal">CREATE MATERIALIZED VIEW</code> which create a new
- table and populate it can use a parallel plan.
- </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="52.2.3. Extended Query">extended query protocol</a>.
- Since <a class="link" href="libpq.html" title="Chapter 33. 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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="parallel-query.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="parallel-plans.html">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">Home</a></td><td width="40%" align="right" valign="top"> 15.3. Parallel Plans</td></tr></table></div></body></html>
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