Re: [HACKERS] propose to pushdown qual into EXCEPT

Tom Lane wrote:
> "Armor" <yupengstone@qq.com> writes:
> >     Because PG does not pushdown qual to the none of the subquery. And I check the source code, find some comments
insrc/backend/optimizer/path/allpaths.c, which says "If the     subquery contains EXCEPT or EXCEPT ALL set ops we
cannotpush quals into it, because that could change the results".
 
> >     However, for this case, I think we can pushdown qual to the  left most subquery of EXCEPT, just like other
databasedoes.
 
> 
> That is not an adequate argument for such a change being okay.  Postgres,
> with its extensible set of datatypes, has to be much more careful about
> the semantic soundness of optimizations than some other DBs do.

Can we use the record_image_ops mechanism here?

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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