Обсуждение: Raising the geqo_threshold default

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Is there any chance we can raise the default geqo_threshold from
its current default of 12? This seems too low, as a modern Postgres
on modern hardware has no problem with 12 table joins. However, I have
seen geqo causing trouble for clients when they hit 12 and get
random (and crappy) query plans. Is the value of 12 based on any
recent measurements? Thanks.

- --
Greg Sabino Mullane greg@turnstep.com
End Point Corporation
PGP Key: 0x14964AC8 200911162121
http://biglumber.com/x/web?pk=2529DF6AB8F79407E94445B4BC9B906714964AC8
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"Greg Sabino Mullane" <greg@turnstep.com> writes:
> Is there any chance we can raise the default geqo_threshold from
> its current default of 12?

We were over that just a few months ago.
        regards, tom lane

On Mon, Nov 16, 2009 at 10:42 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Greg Sabino Mullane" <greg@turnstep.com> writes:
>> Is there any chance we can raise the default geqo_threshold from
>> its current default of 12?
>
> We were over that just a few months ago.

Yeah.  I think we need to see if we can do something about the
ridiculous amount of memory that the standard planner consumes for
large join problems.  I would like to look into this problem, too,
which might be related, but have not had time:

http://archives.postgresql.org/pgsql-hackers/2009-11/msg00328.php

...Robert

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Tom Lane replied:
>> Is there any chance we can raise the default geqo_threshold from
>> its current default of 12?

> We were over that just a few months ago.

Tom, that was a very unhelpful reply. I don't have the luxury of
reading, much less remembering, every thread that has occurred on
the hackers mailing list. Perhaps you mean the discussion from
July 6, 2009 started by Robert Haas? That was approximately 8000
messages ago, and no consensus was reached that I could find. So
I'd like to respectfully ask the hackers to consider raising the
default value from 12 to 16 (as Robert Haas first suggested) or
even higher (20?). Whether or not we make GEQO less random, replace
it someday with something else, tweak *_collapse_limit, etc. is not
as important in my mind as minimizing this unexpected foot gun for
future releases. Thanks.

- --
Greg Sabino Mullane greg@turnstep.com
End Point Corporation
PGP Key: 0x14964AC8 200911170859
http://biglumber.com/x/web?pk=2529DF6AB8F79407E94445B4BC9B906714964AC8
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"Greg Sabino Mullane" <greg@turnstep.com> writes:
> Perhaps you mean the discussion from
> July 6, 2009 started by Robert Haas? That was approximately 8000
> messages ago, and no consensus was reached that I could find.

Precisely.

> So I'd like to respectfully ask the hackers to consider raising the
> default value from 12 to 16 (as Robert Haas first suggested) or
> even higher (20?).

Have you got any evidence whatsoever to back up those suggestions?
(In particular, evidence that it's not going to cause serious
performance degradations for some people.)  Otherwise you're not
advancing the discussion past where it left off.
        regards, tom lane