From: | Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> |
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To: | Kevin Grittner <kgrittn(at)ymail(dot)com>, Michael Kolomeitsev <mkolomeitsev(at)gmail(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Pg makes nonoptimal choice between two multicolumn indexes with the same columns but in different order. |
Date: | 2013-12-28 21:19:20 |
Message-ID: | 52BF4058.6000306@archidevsys.co.nz |
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Lists: | pgsql-performance |
On 29/12/13 10:03, Kevin Grittner wrote:
> Michael Kolomeitsev <mkolomeitsev(at)gmail(dot)com> wrote:
>
>> it is clear for me why t1_b_a_idx is better. The question is: Is
>> postgresql able to see that?
> For a number of reasons I never consider a bulk load complete until
> I run VACUUM FREEZE ANALYZE on the table(s) involved. When I try
> your test case without that, I get the bad index choice. When I
> then run VACUUM FREEZE ANALYZE on the database I get the good index
> choice.
>
> There may be some lesser maintenance which sets up visibility
> information and provides the planner with enough data to make a
> good choice, I just noticed that you were not following what I
> consider to be rote good practice, tried it, and it solved the
> problem.
>
> --
> Kevin Grittner
> EDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>
>
Curious: Would it be feasible to do some kind of ANALYZE during a bulk
operation? Say if you could tell the system you expected to change 20%
of the records in advance: then you could sample some of the changes and
modify the statistics with 0.2 times that plus 0.8 of the pre-existing
statistics.
BEGIN BULK OPERATION CHANGE 20%
[... several transactions ...]
END BULK OPERATION
The sampling could be done as part of the individual operations or at
the end of the bulk operation - whichever is deemed more practicable
(possibly a bit of both?).
Cheers,
Gavin
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