| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
| Cc: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Performance improvement for joins where outer side is unique |
| Date: | 2016-03-12 15:51:39 |
| Message-ID: | 13157.1457797899@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> Don't the semantics of a SEMI JOIN also state that the output columns only
> come from the outer relation? i.e., the inner relation doesn't contribute
> either rows or columns to the final result? Or is that simply
> an implementation artifact of the fact that the only current way to perform
> a semi-join explicitly is via exists/in?
I think it's an artifact. What nodes.h actually says about it is you get
the values of one randomly-selected matching inner row, which seems like
a fine definition for the purposes we plan to put it to.
regards, tom lane
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