doc: Move force_parallel_mode to section for developer options

This GUC has always been classified as a planner option since its
introduction in 7c944bd9, and was listed in postgresql.conf.sample.  As
this parameter exists for testing purposes, move it to the section
dedicated to developer parameters and hence remove it from
postgresql.conf.sample.  This will avoid any temptation to play with it
on production servers for users that should never really have to touch
this parameter.

The general description used for developer options is reworded a bit, to
take into account the inclusion of force_parallel_mode, per a suggestion
from Tom Lane.

Per discussion between Tom Lane, Bruce Momjian, Justin Pryzby, Bharath
Rupireddy and me.

Author: Justin Pryzby, Tom Lane
Discussion: https://postgr.es/m/20210403152402.GA8049@momjian.us

doc/src/sgml/config.sgml

@@ -5912,51 +5912,6 @@ SELECT * FROM parent WHERE key = 2400;
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-force-parallel-mode" xreflabel="force_parallel_mode">
-      <term><varname>force_parallel_mode</varname> (<type>enum</type>)
-      <indexterm>
-       <primary><varname>force_parallel_mode</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        Allows the use of parallel queries for testing purposes even in cases
-        where no performance benefit is expected.
-        The allowed values of <varname>force_parallel_mode</varname> are
-        <literal>off</literal> (use parallel mode only when it is expected to improve
-        performance), <literal>on</literal> (force parallel query for all queries
-        for which it is thought to be safe), and <literal>regress</literal> (like
-        <literal>on</literal>, but with additional behavior changes as explained
-        below).
-       </para>
-
-       <para>
-        More specifically, setting this value to <literal>on</literal> will add
-        a <literal>Gather</literal> node to the top of any query plan for which this
-        appears to be safe, so that the query runs inside of a parallel worker.
-        Even when a parallel worker is not available or cannot be used,
-        operations such as starting a subtransaction that would be prohibited
-        in a parallel query context will be prohibited unless the planner
-        believes that this will cause the query to fail.  If failures or
-        unexpected results occur when this option is set, some functions used
-        by the query may need to be marked <literal>PARALLEL UNSAFE</literal>
-        (or, possibly, <literal>PARALLEL RESTRICTED</literal>).
-       </para>
-
-       <para>
-        Setting this value to <literal>regress</literal> has all of the same effects
-        as setting it to <literal>on</literal> plus some additional effects that are
-        intended to facilitate automated regression testing.  Normally,
-        messages from a parallel worker include a context line indicating that,
-        but a setting of <literal>regress</literal> suppresses this line so that the
-        output is the same as in non-parallel execution.  Also,
-        the <literal>Gather</literal> nodes added to plans by this setting are hidden
-        in <literal>EXPLAIN</literal> output so that the output matches what
-        would be obtained if this setting were turned <literal>off</literal>.
-       </para>
-      </listitem>
-     </varlistentry>
-
      <varlistentry id="guc-plan-cache_mode" xreflabel="plan_cache_mode">
       <term><varname>plan_cache_mode</varname> (<type>enum</type>)
       <indexterm>
@@ -10374,11 +10329,10 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
     <title>Developer Options</title>
 
     <para>
-     The following parameters are intended for work on the
-     <productname>PostgreSQL</productname> source code, and in some cases
-     to assist with recovery of severely damaged databases.  There
-     should be no reason to use them on a production database.
-     As such, they have been excluded from the sample
+     The following parameters are intended for developer testing, and
+     should never be used on a production database.  However, some of
+     them can be used to assist with the recovery of severely damaged
+     databases.  As such, they have been excluded from the sample
      <filename>postgresql.conf</filename> file.  Note that many of these
      parameters require special source compilation flags to work at all.
     </para>
@@ -10464,6 +10418,51 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-force-parallel-mode" xreflabel="force_parallel_mode">
+      <term><varname>force_parallel_mode</varname> (<type>enum</type>)
+      <indexterm>
+       <primary><varname>force_parallel_mode</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Allows the use of parallel queries for testing purposes even in cases
+        where no performance benefit is expected.
+        The allowed values of <varname>force_parallel_mode</varname> are
+        <literal>off</literal> (use parallel mode only when it is expected to improve
+        performance), <literal>on</literal> (force parallel query for all queries
+        for which it is thought to be safe), and <literal>regress</literal> (like
+        <literal>on</literal>, but with additional behavior changes as explained
+        below).
+       </para>
+
+       <para>
+        More specifically, setting this value to <literal>on</literal> will add
+        a <literal>Gather</literal> node to the top of any query plan for which this
+        appears to be safe, so that the query runs inside of a parallel worker.
+        Even when a parallel worker is not available or cannot be used,
+        operations such as starting a subtransaction that would be prohibited
+        in a parallel query context will be prohibited unless the planner
+        believes that this will cause the query to fail.  If failures or
+        unexpected results occur when this option is set, some functions used
+        by the query may need to be marked <literal>PARALLEL UNSAFE</literal>
+        (or, possibly, <literal>PARALLEL RESTRICTED</literal>).
+       </para>
+
+       <para>
+        Setting this value to <literal>regress</literal> has all of the same effects
+        as setting it to <literal>on</literal> plus some additional effects that are
+        intended to facilitate automated regression testing.  Normally,
+        messages from a parallel worker include a context line indicating that,
+        but a setting of <literal>regress</literal> suppresses this line so that the
+        output is the same as in non-parallel execution.  Also,
+        the <literal>Gather</literal> nodes added to plans by this setting are hidden
+        in <literal>EXPLAIN</literal> output so that the output matches what
+        would be obtained if this setting were turned <literal>off</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-ignore-system-indexes" xreflabel="ignore_system_indexes">
       <term><varname>ignore_system_indexes</varname> (<type>boolean</type>)
       <indexterm>

src/backend/utils/misc/guc.c

@@ -4922,10 +4922,10 @@ static struct config_enum ConfigureNamesEnum[] =
 	},
 
 	{
-		{"force_parallel_mode", PGC_USERSET, QUERY_TUNING_OTHER,
+		{"force_parallel_mode", PGC_USERSET, DEVELOPER_OPTIONS,
 			gettext_noop("Forces use of parallel query facilities."),
 			gettext_noop("If possible, run query using a parallel worker and with parallel restrictions."),
-			GUC_EXPLAIN
+			GUC_NOT_IN_SAMPLE | GUC_EXPLAIN
 		},
 		&force_parallel_mode,
 		FORCE_PARALLEL_OFF, force_parallel_mode_options,

src/backend/utils/misc/postgresql.conf.sample

@@ -421,7 +421,6 @@
 #from_collapse_limit = 8
 #join_collapse_limit = 8		# 1 disables collapsing of explicit
 					# JOIN clauses
-#force_parallel_mode = off
 #jit = on				# allow JIT compilation
 #plan_cache_mode = auto			# auto, force_generic_plan or
 					# force_custom_plan