Re: PostgreSQL seems to create inefficient plans in simple conditional joins

On 31 January 2016 at 01:30, Hedayat Vatankhah <hedayat.fwd@gmail.com> wrote:
> Personally, I expect both queries below to perform exactly the same:
>
> SELECT
>     t1.id, *
> FROM
>     t1
> INNER JOIN
>     t2 ON t1.id = t2.id
>     where t1.id > -9223372036513411363;
>
> And:
>
> SELECT
>     t1.id, *
> FROM
>     t1
> INNER JOIN
>     t2 ON t1.id = t2.id
>     where t1.id > -9223372036513411363 and t2.id > -9223372036513411363;
>
> Unfortunately, they do not. PostgreSQL creates different plans for these
> queries, which results in very poor performance for the first one compared
> to the second (What I'm testing against is a DB with around 350 million
> rows in t1, and slightly less in t2).
>
> EXPLAIN output:
> First query: http://explain.depesz.com/s/uauk
> Second query: link: http://explain.depesz.com/s/uQd

Yes, unfortunately you've done about the only thing that you can do,
and that's just include both conditions in the query. Is there some
special reason why you can't just write the t2.id > ... condition in
the query too? or is the query generated dynamically by some software
that you have no control over?

I'd personally quite like to see improvements in this area, and even
wrote a patch [1] which fixes this problem too. The problem I had when
proposing the fix for this was that I was unable to report details
about how many people are hit by this planner limitation. The patch I
proposed caused a very small impact on planning time for many queries,
and was thought by many not to apply in enough cases for it to be
worth slowing down queries which cannot possibly benefit. Of course I
agree with this, I've no interest in slowing down planning on queries,
but at the same time understand the annoying poor optimisation in this
area.

Although please remember the patch I proposed was merely a first draft
proposal. Not for production use.

[1] http://www.postgresql.org/message-id/flat/CAKJS1f9FK_X_5HKcPcSeimy16Owe3EmPmmGsGWLcKkj_rW9s6A@mail.gmail.com#CAKJS1f9FK_X_5HKcPcSeimy16Owe3EmPmmGsGWLcKkj_rW9s6A@mail.gmail.com

--
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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