From: Tom Lane <tgl@sss.pgh.pa.us>
To: Gunnlaugur Thor Briem <gunnlaugur@gmail.com>
cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Jeff Janes <jeff.janes@gmail.com>, Kevin Grittner <kgrittn@ymail.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: EXPLAIN (no ANALYZE) taking an hour for INSERT FROM SELECT
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Comments: In-reply-to Gunnlaugur Thor Briem <gunnlaugur@gmail.com>
 message dated "Wed, 11 Mar 2015 15:54:40 -0000"
Date: Wed, 11 Mar 2015 12:15:33 -0400
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Gunnlaugur Thor Briem <gunnlaugur@gmail.com> writes:
> Yes, I think that's it: I've just realized that immediately prior to the
> INSERT, in the same transaction, an unfiltered DELETE has been issued; i.e.
> the whole table is being rewritten. Then the INSERT is issued ... with a
> WHERE clause on non-existence in the (now empty) table.

> In that case of course the WHERE clause is unnecessary, as it will always
> evaluate as true (and we've locked the whole table for writes). Looks like
> it is a lot worse than unnecessary, though, if it triggers this performance
> snafu in EXPLAIN INSERT.

Ah-hah.  So what's happening is that the planner is doing an indexscan
over the entire table of now-dead rows, looking vainly for an undeleted
maximal row.  Ouch.

I wonder how hard it would be to make the indexscan give up after hitting
N consecutive dead rows, for some suitable N, maybe ~1000.  From the
planner's viewpoint it'd be easy enough to fall back to using whatever
it had in the histogram after all.  But that's all happening down inside
index_getnext, and I'm hesitant to stick some kind of wart into that
machinery for this purpose.

			regards, tom lane


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