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(at)openratings(dot)com]
Sent: Thursday, July 11, 2002 3:51 PM
To: pgsql-general(at)postgresql(dot)org; pgsql-sql(at)postgresql(dot)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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