Adding upstream version 16.2.upstream/16.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
author: Daniel Baumann <daniel.baumann@progress-linux.org> 2024-04-13 13:44:03 +0000
committer: Daniel Baumann <daniel.baumann@progress-linux.org> 2024-04-13 13:44:03 +0000
commit: 293913568e6a7a86fd1479e1cff8e2ecb58d6568 (patch)
tree: fc3b469a3ec5ab71b36ea97cc7aaddb838423a0c /doc/src/sgml/html/jit-decision.html
parent: Initial commit. (diff)
download: postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.tar.xz
postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.zip
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+<?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>32.2. When to JIT?</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="jit-reason.html" title="32.1. What Is JIT compilation?" /><link rel="next" href="jit-configuration.html" title="32.3. Configuration" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">32.2. When to <acronym class="acronym">JIT</acronym>?</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="jit-reason.html" title="32.1. What Is JIT compilation?">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="jit.html" title="Chapter 32. Just-in-Time Compilation (JIT)">Up</a></td><th width="60%" align="center">Chapter 32. Just-in-Time Compilation (<acronym class="acronym">JIT</acronym>)</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="jit-configuration.html" title="32.3. Configuration">Next</a></td></tr></table><hr /></div><div class="sect1" id="JIT-DECISION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">32.2. When to <acronym class="acronym">JIT</acronym>? <a href="#JIT-DECISION" class="id_link">#</a></h2></div></div></div><p>
+   <acronym class="acronym">JIT</acronym> compilation is beneficial primarily for long-running
+   CPU-bound queries. Frequently these will be analytical queries.  For short
+   queries the added overhead of performing <acronym class="acronym">JIT</acronym> compilation
+   will often be higher than the time it can save.
+  </p><p>
+   To determine whether <acronym class="acronym">JIT</acronym> compilation should be used,
+   the total estimated cost of a query (see
+   <a class="xref" href="planner-stats-details.html" title="Chapter 76. How the Planner Uses Statistics">Chapter 76</a> and
+   <a class="xref" href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS" title="20.7.2. Planner Cost Constants">Section 20.7.2</a>) is used.
+   The estimated cost of the query will be compared with the setting of <a class="xref" href="runtime-config-query.html#GUC-JIT-ABOVE-COST">jit_above_cost</a>. If the cost is higher,
+   <acronym class="acronym">JIT</acronym> compilation will be performed.
+   Two further decisions are then needed.
+   Firstly, if the estimated cost is more
+   than the setting of <a class="xref" href="runtime-config-query.html#GUC-JIT-INLINE-ABOVE-COST">jit_inline_above_cost</a>, short
+   functions and operators used in the query will be inlined.
+   Secondly, if the estimated cost is more than the setting of <a class="xref" href="runtime-config-query.html#GUC-JIT-OPTIMIZE-ABOVE-COST">jit_optimize_above_cost</a>, expensive optimizations are
+   applied to improve the generated code.
+   Each of these options increases the <acronym class="acronym">JIT</acronym> compilation
+   overhead, but can reduce query execution time considerably.
+  </p><p>
+   These cost-based decisions will be made at plan time, not execution
+   time. This means that when prepared statements are in use, and a generic
+   plan is used (see <a class="xref" href="sql-prepare.html" title="PREPARE"><span class="refentrytitle">PREPARE</span></a>), the values of the
+   configuration parameters in effect at prepare time control the decisions,
+   not the settings at execution time.
+  </p><div class="note"><h3 class="title">Note</h3><p>
+    If <a class="xref" href="runtime-config-query.html#GUC-JIT">jit</a> is set to <code class="literal">off</code>, or if no
+    <acronym class="acronym">JIT</acronym> implementation is available (for example because
+    the server was compiled without <code class="literal">--with-llvm</code>),
+    <acronym class="acronym">JIT</acronym> will not be performed, even if it would be
+    beneficial based on the above criteria.  Setting <a class="xref" href="runtime-config-query.html#GUC-JIT">jit</a>
+    to <code class="literal">off</code> has effects at both plan and execution time.
+   </p></div><p>
+   <a class="xref" href="sql-explain.html" title="EXPLAIN"><span class="refentrytitle">EXPLAIN</span></a> can be used to see whether
+   <acronym class="acronym">JIT</acronym> is used or not.  As an example, here is a query that
+   is not using <acronym class="acronym">JIT</acronym>:
+</p><pre class="screen">
+=# EXPLAIN ANALYZE SELECT SUM(relpages) FROM pg_class;
+                                                 QUERY PLAN
+-------------------------------------------------------------------------------------------------------------
+ Aggregate  (cost=16.27..16.29 rows=1 width=8) (actual time=0.303..0.303 rows=1 loops=1)
+   -&gt;  Seq Scan on pg_class  (cost=0.00..15.42 rows=342 width=4) (actual time=0.017..0.111 rows=356 loops=1)
+ Planning Time: 0.116 ms
+ Execution Time: 0.365 ms
+(4 rows)
+</pre><p>
+   Given the cost of the plan, it is entirely reasonable that no
+   <acronym class="acronym">JIT</acronym> was used; the cost of <acronym class="acronym">JIT</acronym> would
+   have been bigger than the potential savings. Adjusting the cost limits
+   will lead to <acronym class="acronym">JIT</acronym> use:
+</p><pre class="screen">
+=# SET jit_above_cost = 10;
+SET
+=# EXPLAIN ANALYZE SELECT SUM(relpages) FROM pg_class;
+                                                 QUERY PLAN
+-------------------------------------------------------------------------------------------------------------
+ Aggregate  (cost=16.27..16.29 rows=1 width=8) (actual time=6.049..6.049 rows=1 loops=1)
+   -&gt;  Seq Scan on pg_class  (cost=0.00..15.42 rows=342 width=4) (actual time=0.019..0.052 rows=356 loops=1)
+ Planning Time: 0.133 ms
+ JIT:
+   Functions: 3
+   Options: Inlining false, Optimization false, Expressions true, Deforming true
+   Timing: Generation 1.259 ms, Inlining 0.000 ms, Optimization 0.797 ms, Emission 5.048 ms, Total 7.104 ms
+ Execution Time: 7.416 ms
+</pre><p>
+   As visible here, <acronym class="acronym">JIT</acronym> was used, but inlining and
+   expensive optimization were not. If <a class="xref" href="runtime-config-query.html#GUC-JIT-INLINE-ABOVE-COST">jit_inline_above_cost</a> or <a class="xref" href="runtime-config-query.html#GUC-JIT-OPTIMIZE-ABOVE-COST">jit_optimize_above_cost</a> were also lowered,
+   that would change.
+  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="jit-reason.html" title="32.1. What Is JIT compilation?">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="jit.html" title="Chapter 32. Just-in-Time Compilation (JIT)">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="jit-configuration.html" title="32.3. Configuration">Next</a></td></tr><tr><td width="40%" align="left" valign="top">32.1. What Is <acronym class="acronym">JIT</acronym> compilation? </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.2 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 32.3. Configuration</td></tr></table></div></body></html>
+\ No newline at end of file
author	Daniel Baumann <daniel.baumann@progress-linux.org>	2024-04-13 13:44:03 +0000
committer	Daniel Baumann <daniel.baumann@progress-linux.org>	2024-04-13 13:44:03 +0000
commit	293913568e6a7a86fd1479e1cff8e2ecb58d6568 (patch)
tree	fc3b469a3ec5ab71b36ea97cc7aaddb838423a0c /doc/src/sgml/html/jit-decision.html
parent	Initial commit. (diff)
download	postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.tar.xz postgresql-16-293913568e6a7a86fd1479e1cff8e2ecb58d6568.zip