Allow compute_query_id to be set to 'auto' and make it default

Allowing only on/off meant that all either all existing configuration
guides would become obsolete if we disabled it by default, or that we
would have to accept a performance loss in the default config if we
enabled it by default.  By allowing 'auto' as a middle ground, the
performance cost is only paid by those who enable pg_stat_statements and
similar modules.

I only edited the release notes to comment-out a paragraph that is now
factually wrong; further edits are probably needed to describe the
related change in more detail.

Author: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20210513002623.eugftm4nk2lvvks3@nol

3 files changed, 14 insertions, 11 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 45bd1f1b7e3..7e32b0686c6 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -7627,7 +7627,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
      <variablelist>
 
      <varlistentry id="guc-compute-query-id" xreflabel="compute_query_id">
-      <term><varname>compute_query_id</varname> (<type>boolean</type>)
+      <term><varname>compute_query_id</varname> (<type>enum</type>)
       <indexterm>
        <primary><varname>compute_query_id</varname> configuration parameter</primary>
       </indexterm>
@@ -7643,7 +7643,12 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
         identifier to be computed.  Note that an external module can
         alternatively be used if the in-core query identifier computation
         method is not acceptable.  In this case, in-core computation
-        must be disabled.  The default is <literal>off</literal>.
+        must be always disabled.
+        Valid values are <literal>off</literal> (always disabled),
+        <literal>on</literal> (always enabled) and <literal>auto</literal>,
+        which lets modules such as <xref linkend="pgstatstatements"/>
+        automatically enable it.
+        The default is <literal>auto</literal>.
        </para>
        <note>
         <para>
diff --git a/doc/src/sgml/pgstatstatements.sgml b/doc/src/sgml/pgstatstatements.sgml
index bc2b6038ee8..aa332d8cc22 100644
--- a/doc/src/sgml/pgstatstatements.sgml
+++ b/doc/src/sgml/pgstatstatements.sgml
@@ -18,18 +18,14 @@
   <xref linkend="guc-shared-preload-libraries"/> in
   <filename>postgresql.conf</filename>, because it requires additional shared memory.
   This means that a server restart is needed to add or remove the module.
+  In addition, query identifier calculation must be enabled in order for the
+  module to be active, which is done automatically if <xref linkend="guc-compute-query-id"/>
+  is set to <literal>auto</literal> or <literal>on</literal>, or any third-party
+  module that calculates query identifiers is loaded.
  </para>
 
  <para>
-  The module will not track statistics unless query
-  identifiers are calculated.  This can be done by enabling <xref
-  linkend="guc-compute-query-id"/> or using a third-party module that
-  computes its own query identifiers.  Note that all statistics tracked
-  by this module must be reset if the query identifier method is changed.
- </para>
-
- <para>
-   When <filename>pg_stat_statements</filename> is loaded, it tracks
+   When <filename>pg_stat_statements</filename> is active, it tracks
    statistics across all databases of the server.  To access and manipulate
    these statistics, the module provides views
    <structname>pg_stat_statements</structname> and
diff --git a/doc/src/sgml/release-14.sgml b/doc/src/sgml/release-14.sgml
index c4df401c686..bad12860924 100644
--- a/doc/src/sgml/release-14.sgml
+++ b/doc/src/sgml/release-14.sgml
@@ -3181,10 +3181,12 @@ Author: Bruce Momjian <bruce@momjian.us>
 Move query hash computation from pg_stat_statements to the core server (Julien Rouhaud)
 </para>
 
+<!--
 <para>
 Extension pg_stat_statements will now need to enable query hash computation to function properly.
 This can be done by enabling the server variable compute_query_id or by using an extension with a custom hash computation method.
 </para>
+-->
 </listitem>
 
 <listitem>