Re: EXISTS clauses not being optimized in the face of 'one time pass' optimizable expressions

On Fri, Jul 1, 2016 at 9:52 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Tue, Jun 21, 2016 at 4:18 PM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
>>> explain analyze select * from foo where false or exists (select 1 from
>>> bar where good and foo.id = bar.id); -- A
>>> explain analyze select * from foo where exists (select 1 from bar
>>> where good and foo.id = bar.id); -- B
>>>
>>> These queries are trivially verified as identical but give very different plans.
>
>> Right. I suspect wouldn't be very hard to notice the special case of
>> FALSE OR (SOMETHING THAT MIGHT NOT BE FALSE) but I'm not sure that's
>> worth optimizing by itself.
>
> Constant-folding will get rid of the OR FALSE (as well as actually-useful
> variants of this example). The problem is that that doesn't happen till
> after we identify semijoins. So the second one gives you a semijoin plan
> and the first doesn't. This isn't especially easy to improve. Much of
> the value of doing constant-folding would disappear if we ran it before
> subquery pullup + join simplification, because in non-stupidly-written
> queries those are what expose the expression simplification opportunities.
> We could run it twice but that seems certain to be a dead loser most of
> the time.
>
>> A more promising line of attack as it
>> seems to me is to let the planner transform back and forth between
>> this form for the query and the UNION form.
>
> Maybe, but neither UNION nor UNION ALL would duplicate the semantics
> of OR, so there's some handwaving here that I missed.

SELECT * FROM foo WHERE a = 5 OR a = 4

isn't equivalent to

SELECT * FROM foo WHERE a = 5
UNION
SELECT * FROM foo WHERE a = 4

?

--
Robert Haas
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