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Improve JIT docs.

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Author: John Naylor and Andres Freund
Discussion: https://postgr.es/m/CAJVSVGUs-VcwSY7-Kx-GQe__8hvWuA4Uhyf3gxoMXeiZqebE9g@mail.gmail.com
This commit is contained in:
Andres Freund
2018-03-29 16:13:40 -07:00
parent c1de1a3a8b
commit fb60478011
3 changed files with 56 additions and 55 deletions
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4
doc/src/sgml/func.sgml
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@ -15945,8 +15945,8 @@ SELECT * FROM pg_ls_dir('.') WITH ORDINALITY AS t(ls,n);
<row>
<entry><literal><function>pg_jit_available()</function></literal></entry>
<entry><type>boolean</type></entry>
<entry>is <acronym>JIT</acronym> available in this session (see <xref
linkend="jit"/>)? Returns <literal>false</literal> if <xref
<entry>is <acronym>JIT</acronym> compilation available in this session
(see <xref linkend="jit"/>)? Returns <literal>false</literal> if <xref
linkend="guc-jit"/> is set to false.</entry>
</row>

29
doc/src/sgml/jit.sgml
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@ -18,7 +18,7 @@
</para>
<sect1 id="jit-reason">
<title>What is <acronym>JIT</acronym>?</title>
<title>What is <acronym>JIT</acronym> compilation?</title>
<para>
Just-in-time compilation (<acronym>JIT</acronym>) is the process of turning
@ -33,7 +33,7 @@
<para>
<productname>PostgreSQL</productname> has builtin support to perform
<acronym>JIT</acronym> using <ulink
<acronym>JIT</acronym> compilation using <ulink
url="https://llvm.org/"><productname>LLVM</productname></ulink> when
<productname>PostgreSQL</productname> was built with
<literal>--with-llvm</literal> (see <xref linkend="configure-with-llvm"/>).
@ -97,15 +97,15 @@
<title>When to <acronym>JIT</acronym>?</title>
<para>
<acronym>JIT</acronym> is beneficial primarily for long-running CPU bound
queries. Frequently these will be analytical queries. For short queries
the overhead of performing <acronym>JIT</acronym> will often be higher than
the time it can save.
<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>JIT</acronym> compilation
will often be higher than the time it can save.
</para>
<para>
To determine whether <acronym>JIT</acronym> is used, the total cost of a
query (see <xref linkend="planner-stats-details"/> and <xref
To determine whether <acronym>JIT</acronym> compilation is used, the total
cost of a query (see <xref linkend="planner-stats-details"/> and <xref
linkend="runtime-config-query-constants"/>) is used.
</para>
@ -117,9 +117,9 @@
<para>
If the planner, based on the above criterion, decided that
<acronym>JIT</acronym> is beneficial, two further decisions are
<acronym>JIT</acronym> compilation is beneficial, two further decisions are
made. Firstly, if the query is more costly than the <xref
linkend="guc-jit-optimize-above-cost"/>, GUC expensive optimizations are
linkend="guc-jit-optimize-above-cost"/> GUC, expensive optimizations are
used to improve the generated code. Secondly, if the query is more costly
than the <xref linkend="guc-jit-inline-above-cost"/> GUC, short functions
and operators used in the query will be inlined. Both of these operations
@ -187,8 +187,9 @@ SET
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
</programlisting>
As visible here, <acronym>JIT</acronym> was used, but inlining and
optimization were not. If <xref linkend="guc-jit-optimize-above-cost"/>,
<xref linkend="guc-jit-inline-above-cost"/> were lowered, just like <xref
expensive optimization were not. If <xref
linkend="guc-jit-optimize-above-cost"/>, <xref
linkend="guc-jit-inline-above-cost"/> were lowered, just like <xref
linkend="guc-jit-above-cost"/>, that would change.
</para>
</sect1>
@ -197,8 +198,8 @@ SET
<title>Configuration</title>
<para>
<xref linkend="guc-jit"/> determines whether <acronym>JIT</acronym> is
enabled or disabled.
<xref linkend="guc-jit"/> determines whether <acronym>JIT</acronym>
compilation is enabled or disabled.
</para>
<para>