| From: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Armor <yupengstone(at)qq(dot)com>, pgsql-hackers(at)postgreSQL(dot)org |
| Subject: | Re: propose to pushdown qual into EXCEPT |
| Date: | 2016-12-23 22:24:00 |
| Message-ID: | 20161223222400.ysgmlmv7pj7wh7ph@alvherre.pgsql |
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| Lists: | pgsql-hackers |
Tom Lane wrote:
> "=?ISO-8859-1?B?QXJtb3I=?=" <yupengstone(at)qq(dot)com> writes:
> > Because PG does not pushdown qual to the none of the subquery. And I check the source code, find some comments in src/backend/optimizer/path/allpaths.c, which says "If the subquery contains EXCEPT or EXCEPT ALL set ops we cannot push 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 database does.
>
> 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/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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