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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