TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
Greetz,
Guido
> Cannot you do a cast in your query? Does that help with using the indexes?
>
> -----Original Message-----
> From: pgsql-performance-owner@postgresql.org
> [mailto:pgsql-performance-owner@postgresql.org]On Behalf Of
> mailing@impactmedia.de
> Sent: maandag 2 augustus 2004 14:09
> To: pgsql-performance@postgresql.org
> Subject: [PERFORM] No index usage with "left join"
>
>
> We have a "companies" and a "contacts" table with about 3000 records
> each.
>
> We run the following SQL-Command which runs about 2 MINUTES !:
>
> SELECT count(*) FROM contacts LEFT JOIN companies ON contacts.sid =
> companies.intfield01
>
> contacts.sid (type text, b-tree index on it)
> companies.intfield01 (type bigint, b-tree index on it)
>
> comfire=> explain analyze SELECT count(*) FROM prg_contacts LEFT JOIN
> prg_addresses ON prg_contacts.sid=prg_addresses.intfield01;
> NOTICE: QUERY PLAN:
>
> Aggregate (cost=495261.02..495261.02 rows=1 width=15) (actual
> time=40939.38..40939.38 rows=1 loops=1)
> -> Nested Loop (cost=0.00..495253.81 rows=2885 width=15) (actual
> time=0.05..40930.14 rows=2866 loops=1)
> -> Seq Scan on prg_contacts (cost=0.00..80.66 rows=2866
> width=7) (actual time=0.01..18.10 rows=2866 loops=1)
> -> Seq Scan on prg_addresses (cost=0.00..131.51 rows=2751
> width=8) (actual time=0.03..6.25 rows=2751 loops=2866)
> Total runtime: 40939.52 msec
>
> EXPLAIN
>
> Note:
> - We need the left join because we need all contacts even if they are
> not assigned to a company
> - We are not able to change the datatypes of the joined fields
> because we use a standard software (btw who cares: SuSE Open Exchange
> Server)
> - When we use a normal join (without LEFT or a where clause) the SQL
> runs immediately using the indexes
>
> How can I force the usage of the indexes when using "left join". Or
> any other SQL construct that does the same !? Can anybody please give
> us a hint !?
>
> Thanks in forward.
>
> Greetings
> Achim
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
> joining column's datatypes do not match
>
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
> joining column's datatypes do not match
