Re: from_collapse_limit vs. geqo_threshold

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Jun 1, 2009 at 3:34 PM, Selena Deckelmann <selena(at)endpoint(dot)com> wrote:
>> Suggested revision of Robert's prose:
>>
>> Because genetic query optimization may be triggered, increasing
>> from_collapse_limit should be considered relative to <xref
>> linkend="guc-geqo-threshold">.

> Here's my attempt.

I applied the attached, along with some other wordsmithing.

regards, tom lane

***************
*** 2252,2261 ****
The planner will merge sub-queries into upper queries if the
resulting <literal>FROM</literal> list would have no more than
this many items. Smaller values reduce planning time but might
! yield inferior query plans. The default is eight. It is usually
! wise to keep this less than <xref linkend="guc-geqo-threshold">.
For more information see <xref linkend="explicit-joins">.
</para>
</listitem>
</varlistentry>

--- 2261,2275 ----
The planner will merge sub-queries into upper queries if the
resulting <literal>FROM</literal> list would have no more than
this many items. Smaller values reduce planning time but might
! yield inferior query plans. The default is eight.
For more information see <xref linkend="explicit-joins">.
</para>
+
+ <para>
+ Setting this value to <xref linkend="guc-geqo-threshold"> or more
+ may trigger use of the GEQO planner, resulting in nondeterministic
+ plans. See <xref linkend="runtime-config-query-geqo">.
+ </para>
</listitem>
</varlistentry>