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Re: Incorrect row estimates in plan?

From: pgdba <postgresql(at)inbox(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Incorrect row estimates in plan?
Date: 2007-09-26 15:24:01
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Hi Tom,

Tom Lane-2 wrote:
> pgdba <postgresql(at)inbox(dot)com> writes:
>>             ->  Bitmap Heap Scan on slog  (cost=82.98..6434.62 rows=2870
>> width=61) (actual time=50.235..1237.948 rows=83538 loops=1)
>>                     Recheck 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))
>>                     ->  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?
> With such a bizarre scan condition, it's unlikely you'll get any really
> accurate row estimate.
> 			regards, tom lane

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). It is using the destip when rule 8/9, and
srcip when other, but only for a subset of the rules (1,2,8,9,10). There are
about 18 rules in total, but I'm only interested in those 5. I have tried a
couple of indices like:
create index test_destip_idx on slog (gid,destip) where rule in (8,9);
create index test_srcip_idx on slog (gid,srcip) where rule in (1,2,10);

But the original slog_gri_idx index was used instead. Is there a way that I
can rewrite that index then? Not that I'm a fan of a CASE statement in a
functional index, but I'm at a loss as to how else I can create this. Or
what else I can look into to make this faster?

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Subject: Re: Incorrect row estimates in plan?
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Subject: Re: Searching for the cause of a bad plan

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