On Mar 3, 2009, at 3:26 PM, Emi Lu wrote:
Good morning,
Would it be possible to know that whether "union" is quicker than
"left join" in a general sense?
Queries:
========================
(1) union
select a.*, b.value
from a
left join b
union
select a.*, c.value
from a
left join c
(2) left join
select distinct a.*,
case when ... then
b.value
else
c.value
from a
left join b
left join c
Will (1) be more efficient than (2) or vice versa? Or it really
depends on data
Thanks a lot!
Lu Ying
I would say try it in your situation...
I never did any real worl testing, but usually I grab a left join
before anything else.
I would suspect that the planner knows a bit better how to optimize
the left join version.
Ries
--
Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-sql
