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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>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><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">32.2. When to <acronym xmlns="http://www.w3.org/1999/xhtml" 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 xmlns="http://www.w3.org/1999/xhtml" class="acronym">JIT</acronym>)</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 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></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>?</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 70. How the Planner Uses Statistics">Chapter 70</a> and
<a class="xref" href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS" title="19.7.2. Planner Cost Constants">Section 19.7.2</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)
-> 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)
-> 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.
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