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Re: Incorrect row estimates in plan?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgdba <postgresql(at)inbox(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Incorrect row estimates in plan?
Date: 2007-09-26 16:38:09
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
pgdba <postgresql(at)inbox(dot)com> writes:
> Tom Lane-2 wrote:
> ->  Bitmap Index Scan on slog_gri_idx 
> (cost=0.00..82.26
> rows=2870 width=0) (actual time=41.306..41.306 rows=83538 loops=1)
> Index Cond: ((gid = 10000) AND (rule = ANY
> ('{1,2,8,9,10}'::integer[])) AND (CASE WHEN (rule = ANY
> ('{8,9}'::integer[])) THEN destip ELSE srcip END =
> ''::inet))
>> [ blink... ]  Pray tell, what is the definition of this index?

> Original index: "create index slog_gri_idx on slog (gid,rule,(case when rule
> in (8,9) then
> destip else srcip end)) WHERE (rule in (1, 2, 8, 9, 10))"

> The purpose of that index is to match a specific query (one that gets run
> frequently and needs to be fast).

Ah.  I didn't think you would've put such a specific thing into an index
definition, but if you're stuck supporting such badly written queries,
maybe there's no other way.

I rather doubt that you're going to be able to make this query any
faster than it is, short of buying enough RAM to keep the whole table
RAM-resident.  Pulling 80000 random rows in 1200 msec doesn't sound
all that slow to me.

The ultimate solution might be to rethink your table designs ...

			regards, tom lane

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