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Re: Hashaggregate estimates

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Jorge Montero" <jorge_montero(at)homedecorators(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Hashaggregate estimates
Date: 2010-01-13 22:08:15
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Lists: pgsql-performance
"Jorge Montero" <jorge_montero(at)homedecorators(dot)com> writes:
> The killer seems to be the row aggregation. There are about 95K
> different values of sku_id in the sales table, and even the best
> seller items are a very small percentage of all rows, so expecting the
> aggregation to consolidate the rows 50:1 like it does in one of the
> explains above is a pipe dream. I've increased statistics in sku_id
> into the three digits, but results are not any better

Yeah, estimating the number of distinct values from a sample of the data
is a hard problem :-(.
> Is there any way I can coax Postgres into making a more realistic
> aggregation estimate?

There's no good way in 8.3.  (In CVS HEAD there's a feature to manually
override the ndistinct estimate for a column.)  In principle you could
manually update the pg_statistic.stadistinct value for the column, but
the trouble with that is the next ANALYZE will overwrite it.

			regards, tom lane

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