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Re: Thoughts on statistics for continuously advancing columns

From: Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Josh Berkus <josh(at)agliodbs(dot)com>, Nathan Boley <npboley(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Thoughts on statistics for continuously advancing columns
Date: 2009-12-31 04:17:29
Message-ID: 4B3C25D9.2010904@postnewspapers.com.au (view raw or flat)
Thread:
Lists: pgsql-hackers
On 31/12/2009 12:33 AM, Kevin Grittner wrote:
> Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us>  wrote:
>
>> Well, the problem Josh has got is exactly that a constant high
>> bound doesn't work.
>
> I thought the problem was that the high bound in the statistics fell
> too far below the actual high end in the data.  This tends (in my
> experience) to be much more painful than an artificially extended
> high end in the statistics.  (YMMV, of course.)
>
>> What I'm wondering about is why he finds that re-running ANALYZE
>> isn't an acceptable solution.  It's supposed to be a reasonably
>> cheap thing to do.
>
> Good point.  We haven't hit this problem in PostgreSQL precisely
> because we can run ANALYZE often enough to prevent the skew from
> becoming pathological.

While regular ANALYZE seems to be pretty good ... is it insane to 
suggest determining the min/max bounds of problem columns by looking at 
a btree index on the column in ANALYZE, instead of relying on random 
data sampling? An ANALYZE that didn't even have to scan the indexes but 
just look at the ends might be something that could be run much more 
frequently with less I/O and memory cost than a normal ANALYZE, just to 
selectively update key stats that are an issue for such continuously 
advancing columns.

--
Craig Ringer

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