Re: Vanishing table in 188.8.131.52
Date: Thu, 2 Feb 2012 18:29:51 -0000
I'm not sure you're interpreting push_subq correctly.
from parent, child where child.id_parent = parent_id and parent.type = 'RARE' and exists ( select null from ref_p where ref_p.id = parent.id )
Assume that parent.type is indexed and 'RARE' is a value that makes the optimizer drive from the parent table.
We can see that it would make sense to get a parent row, run the subquery to check for existence, then acquire the child rows. The default (historical) Oracle position is to get a parent, join to child, and then run the subquery for each row in that result set. By pushing the subquery (down the parse tree) we make Oracle run the subquery as soon as it has got a row from the parent table.
-- Regards Jonathan Lewis http://jonathanlewis.wordpress.com Oracle Core (Apress 2011) http://www.apress.com/9781430239543 "Mladen Gogala" <gogala.mladen_at_gmail.com> wrote in message news:jgeik2$ve0$1_at_solani.org...Received on Thu Feb 02 2012 - 12:29:51 CST
| On Thu, 02 Feb 2012 14:00:22 +0000, Jonathan Lewis wrote:
| > You're thinking of the undocumented /*+ precompute_subquery */ hint.
| To tell the truth, that does look very similar to push_subq. With non-
| correlated subqueries, the result is probably very similar, in both
| cases. I don't think that it is possible to pre-compute a correlated