From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Dimitri Fontaine <dim(at)hi-media(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: *_collapse_limit, geqo_threshold |
Date: | 2009-07-07 22:15:13 |
Message-ID: | 29789.1247004913@sss.pgh.pa.us |
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Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Tom Lane escribi:
>> My own thought is that from_collapse_limit has more justification,
>> since it basically acts to stop a subquery from being flattened when
>> that would make the parent query too complex, and that seems like a
>> more understandable and justifiable behavior than treating JOIN
>> syntax specially.
> Isn't that what we use OFFSET 0 for? That one has also the nice
> property that you can actually specify which subquery you want to
> prevent from being flattened.
Well, if you want to modify your queries to prevent long planning times,
that'd be one way to do it. It doesn't seem like a generally useful
answer to me though. For example, typically the subquery would actually
be a view that might be used in various contexts. If you stick an
OFFSET in it then you disable flattening in all those contexts, likely
not the best answer.
> Personally I have never seen a case where the collapse_limits were
> useful tools.
I'm not convinced they're useful either.
regards, tom lane
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