Thread: Query planner ignoring constraints on partitioned tables when joining

Michael Okner <michael.okner@gmail.com> writes:
> I've been able to reproduce the issue in a generic environment and posted the code to create this environment on my
GitHubat https://github.com/mikeokner/pgsql_test. The query plans demonstrating this issue are pasted here:
http://bpaste.net/show/92138/.I've poked around on IRC and no one seems to think this is normal behavior. Is it in fact
abug or is there something I should be doing to fix this behavior? 

It's not a bug, though I can see why you'd like to wish it was.

What you've essentially got is

WHERE
    (group_bbb_one.start_time = group_bbb_two.start_time)
    AND
    (group_bbb_one.start_time >= '2013-02-04 00:00:00'
     AND group_bbb_one.start_time < '2013-02-05 00:00:00');

where the first clause is expanded out from the NATURAL JOIN, and the
rest is the way the parser interprets the references to the natural
join's outputs.  So you have fixed constraints only on
group_bbb_one.start_time, which is why constraint exclusion triggers for
that table hierarchy and not the other one.

The only convenient way to fix this is to explicitly repeat the
constraints for each side of the join, eg

SELECT * FROM group_bbb_one NATURAL JOIN group_bbb_two
WHERE (group_bbb_one.start_time >= '2013-02-24 00:00:00'
       AND group_bbb_one.start_time < '2013-02-25 00:00:00')
  AND (group_bbb_two.start_time >= '2013-02-24 00:00:00'
       AND group_bbb_two.start_time < '2013-02-25 00:00:00');

Now I can see why you might think this is a bug, because you don't have
to do it when the WHERE constraint is a simple equality.  Then you
would have, in effect,

WHERE
    (group_bbb_one.start_time = group_bbb_two.start_time)
    AND
    (group_bbb_one.start_time = '2013-02-04 00:00:00');

which the planner's equivalence-class mechanism replaces with

WHERE
    (group_bbb_one.start_time = '2013-02-04 00:00:00')
    AND
    (group_bbb_two.start_time = '2013-02-04 00:00:00');

and so you get fixed constraints on both tables without having to write
it out explicitly.  But that only works for equality conditions.

One could imagine adding planner logic that would make inferences of a
similar sort for equalities combined with inequalities, but it would be
vastly more complicated, and would provide useful results in vastly
fewer queries, than the equality-propagation logic.  So don't hold your
breath waiting for something like that to happen.

            regards, tom lane

On 18 April 2013 22:42, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> One could imagine adding planner logic that would make inferences of a
> similar sort for equalities combined with inequalities, but it would be
> vastly more complicated, and would provide useful results in vastly
> fewer queries, than the equality-propagation logic.  So don't hold your
> breath waiting for something like that to happen.

I'll take note that we need to make partitioning work for merge joins also.

On a more general note, it would be good to be able to look at the
starting value from the driving table of the join and use that as a
constraint in the scan on the second table. We rely on that mechanism
for nested loop joins, so we could do with that here also.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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