From: Anthony Presley <anthony@resolution.com>
To: Pgsql-Performance ((((E-mail)))) <pgsql-performance@postgresql.org>
Subject: Re: Why is it choosing a different plan?
Date: Fri, 22 Sep 2006 18:58:53 -0500
Message-ID: <1158969533.8070.19.camel@puma> (raw)
In-Reply-To: <1158965956.8070.12.camel@puma>
References: <1158965956.8070.12.camel@puma>
I thought this was related to the TYPE (ie, I could cast it using
something like: attr1=1::int8). However, I tried a few more values, and
the query planner is confusing me.
With these values, in the owner, I get a Seq Scan:
'GIL', '1122', '2305'
With these values, in the owner, I get an Index Scan:
'p1', 'p2', '2300', '8088', 'CHANGEINVENTION'
The os_currentstep table has about 119,700 rows in it -- and I can't do
too much to actually change the query, since it's coming from something
of a 'black box' application.
Thoughts?
--
Anthony
On Fri, 2006-09-22 at 17:59 -0500, Anthony Presley wrote:
> Hi all, I'm having some confusion with the 7.4 query planner.
>
> I have two identical queries, whereby the passed (varchar) parameter
> appears to be the deciding factor between a sequential or an index scan.
>
>
> IE, This query:
>
> explain SELECT DISTINCT (a1.ENTRY_ID) AS retrieved FROM OS_CURRENTSTEP
> AS a1 , OS_CURRENTSTEP AS a2 WHERE a1.ENTRY_ID = a1.ENTRY_ID AND
> a1.ENTRY_ID = a2.ENTRY_ID AND ( a1.OWNER = 'p1' AND a2.STEP_ID =
> 1 );
> NOTICE: QUERY PLAN:
>
> Unique (cost=1175.88..1175.88 rows=1 width=16)
> -> Sort (cost=1175.88..1175.88 rows=1 width=16)
> -> Nested Loop (cost=0.00..1175.87 rows=1 width=16)
> -> Index Scan using idx_9 on os_currentstep a1
> (cost=0.00..1172.45 rows=1 width=8)
> -> Index Scan using idx_8 on os_currentstep a2
> (cost=0.00..3.41 rows=1 width=8)
>
> However, this query:
>
> explain SELECT DISTINCT (a1.ENTRY_ID) AS retrieved FROM OS_CURRENTSTEP
> AS a1 , OS_CURRENTSTEP AS a2 WHERE a1.ENTRY_ID = a1.ENTRY_ID AND
> a1.ENTRY_ID = a2.ENTRY_ID AND ( a1.OWNER = 'GIL' AND a2.STEP_ID =
> 1 );
> NOTICE: QUERY PLAN:
>
> Unique (cost=3110.22..3110.22 rows=1 width=16)
> -> Sort (cost=3110.22..3110.22 rows=1 width=16)
> -> Nested Loop (cost=0.00..3110.21 rows=1 width=16)
> -> Seq Scan on os_currentstep a1 (cost=0.00..3106.78
> rows=1 width=8)
> -> Index Scan using idx_8 on os_currentstep a2
> (cost=0.00..3.41 rows=1 width=8)
>
>
> Thoughts about why changing OWNER from 'p1' to 'GIL' would go from an
> Index Scan to a Sequential?
>
> [There is an index on os_currentstep, and it was vacuum analyze'd
> recently.]
>
> Running version 7.4 (working on upgrading to 8.0 soon). Thanks!
>
> --
> Anthony
>
>
> ---------------------------(end of broadcast)---------------------------
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>
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To: pgsql-performance@postgresql.org
Cc: anthony@resolution.com
Subject: Re: Why is it choosing a different plan?
In-Reply-To: <1158969533.8070.19.camel@puma>
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