Thread: Re: [SQL] Please, HELP! Why is the query plan so wrong???

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


-----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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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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>