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doc: Move force_parallel_mode to section for developer options 

This GUC has always been classified as a planner option since its
introduction in 7c944bd, 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
This commit is contained in:
Michael Paquier 2021-04-14 15:55:55 +09:00
parent 93f4146144
commit ac725ee0f9
3 changed files with 51 additions and 53 deletions
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99
doc/src/sgml/config.sgml
View File

@ -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>

4
src/backend/utils/misc/guc.c
View File

@ -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,

1
src/backend/utils/misc/postgresql.conf.sample
View File

@ -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