From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: WIP: Upper planner pathification |
Date: | 2016-03-07 16:09:22 |
Message-ID: | 21803.1457366962@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> The currently-committed code generates paths where nested loops and
> hash joins get pushed beneath the Gather node, but does not generate
> paths where merge joins have been pushed beneath the Gather node. And
> the reason I didn't try to generate those paths is because I believe
> they will almost always suck.
That's a perfectly reasonable engineering judgment (and especially so
for a first release). What I'd really like to see documented is how
that conclusion is related, or not, to the rules about how path nodes
should be decorated with parallel_safe, parallel_degree, etc annotations.
The existing documentation is barely adequate to explain what those fields
mean for primitive scan nodes; it's impossible for anyone but you to
know what they are supposed to mean for joins and higher-level nodes.
regards, tom lane
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