pg.ddx.io pgsql-hackers@postgresql.org mailing list archive help / color / mirror / Atom feed From: Tom Lane <tgl@sss.pgh.pa.us>
To: David G. Johnston <david.g.johnston@gmail.com>
Cc: David Rowley <david.rowley@2ndquadrant.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Performance improvement for joins where outer side is unique
Date: Sat, 12 Mar 2016 10:51:39 -0500
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"David G. Johnston" <david.g.johnston@gmail.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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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, david.g.johnston@gmail.com, david.rowley@2ndquadrant.com, tomas.vondra@2ndquadrant.com, simon@2ndquadrant.com
Subject: Re: Performance improvement for joins where outer side is unique
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