From: | Neil Conway <neilc(at)samurai(dot)com> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: left-deep plans? |
Date: | 2005-02-22 06:40:40 |
Message-ID: | 421AD3E8.7050403@samurai.com |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> Yes, and it's been rejected. The notion is obviously bogus; it amounts
> to assuming that every database is a star schema with only one core table.
Interesting; yes, I suppose that's true.
> Once we get into GEQO territory, we are using the left-deep-only
> heuristic because that's the only kind of plan GEQO can construct.
> But at that point you've already given up any notion of exhaustive
> search.
I think most applications would prefer an exhaustive, deterministic
search of a subset of the search space over a non-exhaustive,
non-deterministic search of the same subset, given approximately the
same performance. In other words, if confining the search to left-deep
plans allows people to use the normal planner in situations where they
would normally be forced to use GEQO to get acceptable performance, I
think that would be a win.
Speaking of which, why does GEQO restrict its search to left-deep plans
only?
-Neil
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