From: Dmitry Tkach <dmitry@openratings.com>
To: Jie Liang <jie@stbernard.com>
Cc: pgsql-general@postgresql.org
Cc: pgsql-sql@postgresql.org
Subject: Re: [SQL] Please, HELP! Why is the query plan so wrong???
Date: Fri, 12 Jul 2002 10:34:27 -0400
Message-ID: <3D2EE8F3.5060904@openratings.com> (raw)
References: <7C760DAA511DC74B99E7D22189F786F101BF211C@MAIL01.stbernard.com>
Jie Liang wrote:
>I believe that SQL will use the index of join 'key' when you join the tables
>if
>have any, in your query the (a,c) is the join key but d is not.
>
>
>Jie Liang
>
Not really... I tried this:
explain select * from fb joing fbr on (fb.a=fbr.a and
fb.c=fbr.c and fbr.d is null) where fb.b=0
It results in the same query plan (seq scan on fbr).
Dima
>
>
>
>-----Original Message-----
>From: Dmitry Tkach [mailto:dmitry@openratings.com]
>Sent: Thursday, July 11, 2002 3:51 PM
>To: pgsql-general@postgresql.org; pgsql-sql@postgresql.org
>Subject: [SQL] Please, HELP! Why is the query plan so wrong???
>
>
>Hi, everybody!
>
>Here is the problem:
>
>test=# create table fb (a int, b int, c datetime);
>CREATE
>test=# create table fbr (a int, c datetime, d int);
>CREATE
>test=# create unique index fb_idx on fb(b);
>CREATE
>test=# create index fbr_idx on fbr(a,c) where d is null;
>CREATE
>test=# set enable_seqscan=off;
>
>SET VARIABLE
>rapidb=# explain select * from fb, fbr where fb.b=0 and fb.a=fbr.a and
>fb.c=fbr.c and fbr.d is null;
>NOTICE: QUERY PLAN:
>
>Hash Join (cost=100000005.82..100001015.87 rows=1 width=32)
> -> Seq Scan on fbr (cost=100000000.00..100001010.00 rows=5 width=16)
> -> Hash (cost=5.81..5.81 rows=1 width=16)
> -> Index Scan using fb_idx on fb (cost=0.00..5.81 rows=1
>width=16)
>
>Could someone PLEASE explain to me, why doesn't it want to use the index on
>fbr?
>
>If I get rid of the join, then it works:
>
>test=# explain select * from fbr where a=1 and c=now() and d is null;
>NOTICE: QUERY PLAN:
>
>Index Scan using fbr_idx on fbr (cost=0.00..5.82 rows=1 width=16)
>
>What's the catch???
>
>Any help would be greatly appreciated!
>
>Thanks!
>
>Dima
>
>
>
>
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To: pgsql-sql@postgresql.org
Cc: dmitry@openratings.com, jie@stbernard.com, pgsql-general@postgresql.org
Subject: Re: [SQL] Please, HELP! Why is the query plan so wrong???
In-Reply-To: <3D2EE8F3.5060904@openratings.com>
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