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
