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Re: Index performance

From: "Brian Modra" <epailty(at)googlemail(dot)com>
To: "Andrew Sullivan" <ajs(at)crankycanuck(dot)ca>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Index performance
Date: 2008-01-03 05:11:07
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Lists: pgsql-hackers
Thanks, I think you have me on the right track. I'm testing a vacuum
analyse now to see how long it takes, and then I'll set it up to
automatically run every night (so that it has a chance to complete
before about 6am.)

On 02/01/2008, Andrew Sullivan <ajs(at)crankycanuck(dot)ca> wrote:
> On Wed, Jan 02, 2008 at 05:53:35PM +0200, Brian Modra wrote:
> > This table is added to in real time, at least 10 rows per second.
> [. . .]
> > If I do a select which uses the pkey index, where equal to the ID
> > column, and greater than one of the values, which should return about
> > 1500 rows, it sometimes takes 1/2 minute to return, and other times
> > takes only seconds.
> >
> > Is it the number of rows being added in real time, that is maybe
> > causing the index to be locked?
> No, it's probably a bad plan.  A minimum 10 rows/second is probably just
> making the statistics for the table look bad.  You likely want to SET
> STATISTICS wider on the 1st (~150 distinct values) column, and then run
> ANALYSE on the table very frequently.  Are you updating or deleting at all?
> If so, that will also affect things: you need to perform very frequent
> VACUUM on that table in that case.
> Aside from that generic advice, it's impossible to say more without EXPLAIN
> ANALYSE output for the slow and fast examples.
> A
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Brian Modra   Land line: +27 23 5411 462
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