Re: small table left outer join big table

----- Original Message ----- 
From: "Alvaro Herrera" <alvherre@commandprompt.com>
To: "Robert Haas" <robertmhaas@gmail.com>
Cc: "Jie Li" <jay23jack@gmail.com>; "pgsql-hackers" <pgsql-hackers@postgresql.org>
Sent: Wednesday, December 29, 2010 8:39 PM
Subject: Re: [HACKERS] small table left outer join big table


> Excerpts from Robert Haas's message of mié dic 29 09:17:17 -0300 2010:
>> On Tue, Dec 28, 2010 at 5:13 AM, Jie Li <jay23jack@gmail.com> wrote:
>> > Hi,
>> >
>> > Please see the following plan:
>> >
>> > postgres=# explain select * from small_table left outer join big_table using
>> > (id);
>> > QUERY PLAN
>> > ----------------------------------------------------------------------------
>> > Hash Left Join (cost=126408.00..142436.98 rows=371 width=12)
>> > Hash Cond: (small_table.id = big_table.id)
>> > -> Seq Scan on small_table (cost=0.00..1.09 rows=9 width=8)
>> > -> Hash (cost=59142.00..59142.00 rows=4100000 width=8)
>> > -> Seq Scan on big_table (cost=0.00..59142.00 rows=4100000
>> > width=8)
>> > (5 rows)
>> >
>> > Here I have a puzzle, why not choose the small table to build hash table? It
>> > can avoid multiple batches thus save significant I/O cost, isn't it?
>> 
>> Yeah, you'd think.  Can you post a full reproducible test case?
> 
> Also, what version is this?
> 
> -- 
> Álvaro Herrera <alvherre@commandprompt.com>
> The PostgreSQL Company - Command Prompt, Inc.
> PostgreSQL Replication, Consulting, Custom Development, 24x7 support

The version is 9.0.1.  I believe the latest version works in the same way.

Thanks,
Li Jie