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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 16:56:33
Message-ID: 12905186.post@talk.nabble.com (view raw or flat)
Thread:
Lists: pgsql-performance


Tom Lane-2 wrote:
> 
> 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 =
>> '192.168.10.23'::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
> 

Badly written the query may be, but I do have the opportunity to change it.
Part of the problem is that I cannot come up with a better way of writing
it.

What about the discrepancy between the estimated row count and the actual
row count for that index access?
"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)"

Is there anything I can do to influence that (not that it is likely to
change the plan, but...). I vacuumed and analyzed after I created the index,
so the stats should be at least be close (with stats target set to 1000
there).

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Subject: Difference in query plan when using = or > in where clause
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