From: Bruce Momjian <bruce@momjian.us>
To: Chris Ruprecht <chris@cdrbill.com>
Cc: Evgeny Shishkin <itparanoia@gmail.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Have: Seq Scan - Want: Index Scan - what am I doing wrong?
Date: Tue, 16 Oct 2012 20:31:06 -0400
Message-ID: <20121017003106.GI7480@momjian.us> (raw)
In-Reply-To: <2A64CF51-CC89-4A0A-89CC-49D7ABFD4603@cdrbill.com>
References: <91417DD6-C9FF-40EA-8EA9-87DF7E696927@cdrbill.com>
<9BB26B73-2824-4C97-92B5-5E059108C071@gmail.com>
<2A64CF51-CC89-4A0A-89CC-49D7ABFD4603@cdrbill.com>
On Tue, Oct 16, 2012 at 08:19:43PM -0400, Chris Ruprecht wrote:
>
> On Oct 16, 2012, at 20:01 , Evgeny Shishkin <itparanoia@gmail.com> wrote:
>
> > Selecting 5 yours of data is not selective at all, so postgres decides it is cheaper to do seqscan.
> >
> > Do you have an index on patient.dnsortpersonnumber? Can you post a result from
> > select count(*) from patient where dnsortpersonnumber = '347450'; ?
> >
>
> Yes, there is an index:
>
> "Aggregate (cost=6427.06..6427.07 rows=1 width=0)"
> " -> Index Scan using patient_pracsortpatientnumber on patient (cost=0.00..6427.06 rows=1 width=0)"
> " Index Cond: (dnsortpersonnumber = '347450'::text)"
>
>
> In fact, all the other criteria is picked using an index. I fear that the >= and <= on the timestamp is causing the issue. If I do a "=" of just one of them, I get an index scan. But I need to scan the entire range. I get queries like "give me everything that was entered into the system for this patient between these two dates". A single date wouldn't work.
Have you read our FAQ on this matter?
http://wiki.postgresql.org/wiki/FAQ#Why_are_my_queries_slow.3F_Why_don.27t_they_use_my_indexes.3F
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
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To: pgsql-performance@postgresql.org
Cc: bruce@momjian.us, chris@cdrbill.com, itparanoia@gmail.com
Subject: Re: Have: Seq Scan - Want: Index Scan - what am I doing wrong?
In-Reply-To: <20121017003106.GI7480@momjian.us>
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