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Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/config.sgml | 45 |
1 files changed, 45 insertions, 0 deletions
diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 392eb700b0c..de84b777300 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -3802,6 +3802,51 @@ 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</> 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</> are + <literal>off</> (use parallel mode only when it is expected to improve + performance), <literal>on</> (force parallel query for all queries + for which it is thought to be safe), and <literal>regress</> (like + on, but with additional behavior changes to facilitate automated + regression testing). + </para> + + <para> + More specifically, setting this value to <literal>on</> will add + a <literal>Gather</> 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</> has all of the same effects + as setting it to <literal>on</> plus some additional effect that are + intended to facilitate automated regression testing. Normally, + messages from a parallel worker are prefixed with a context line, + but a setting of <literal>regress</> suppresses this to guarantee + reproducible results. Also, the <literal>Gather</> nodes added to + plans by this setting are hidden from the <literal>EXPLAIN</> output + so that the output matches what would be obtained if this setting + were turned <literal>off</>. + </para> + </listitem> + </varlistentry> + </variablelist> </sect2> </sect1> |