Re: Terrible plan for join to nested union - Mailing list pgsql-performance

Thanks for your reply Tom.

>> I have a query which joins to a nested union and I'm getting a plan which never returns.  Here is the query
simplifiedas much as possible: 
>> select 'anything' as result
>>                from "Attribute" as A1
>>                               inner join
>>                               (
>>                                              select R."TargetID" as "SourceID"
>>                                                             from "Relationship" as R
>>                                              union
>>                                              select A2."PersonID" as "SourceID"
>>                                                             from "Attribute" as A2
>>                               ) as X on (A1."PersonID" = X."SourceID")
>>                where (A1."ID" = 124791200)
>
>  AFAICS, the UNION result must include every possible value of Attribute.PersonID, which means the inner join cannot
>eliminate any rows of A1 (except those with null PersonID), which seems a tad silly.

It seems to me that the join condition (and hence the restriction) should be pushed down into both sides of the union
tobring the cardinality limit from millions to 1.  I'm imagining a rewrite like this:   
    R(a) J (b U c)  ->  (b J R(a)) U (c J R(a))
...where R = Restrict, J = Join, U = Union

This is the kind of rewrite I would make as a sentient being and it's one that at least one other DBMS I know of makes.

As an aside, even though not as good as pushing down the restriction, the plan that the "union all" produces is decent
performance-wise:
http://explain.depesz.com/s/OZq
It seems to me that a similar alternative could be applied for a distinct union by using two Index Scans followed by a
MergeJoin. 

>What exactly are you trying to accomplish here?

I state in my post that there are several ways to rewrite the query to work-around the issue; I'm not really asking for
awork-around but a) wondering why the plan is so bad; and b) asking if it could be fixed if possible.  Unfortunately
rewritingthe query isn't a trivial matter in our case because the X (union) part of the query is represented logically
asa view, which is expected to be restricted and/or joined so as not to actually materialize the actual union.
Unfortunatelythe PostgreSQL planner seems to want to actually materialize that view.  Working around this would
basicallyentail not using the view, which is used all over the place, and instead duplicating the view's logic except
pushingthe restrictions and/or joins down into both sides of the union in each case.  I could do that, but doing so
wouldbe: a) against the spirit of the Relational Model; b) against the spirit of "fix the planner rather than add
optimizerhints"; c) a royal pain because it causes a rewrite of application logic; d) a point for at least one other
DBMS'soptimizer.  :-) 

>Anyway, I wonder whether you'd get better results with an EXISTS over a correlated UNION ALL subquery, ie, something
like
> ...

Thanks for the work-arounds, but again, that's not quite what I'm after.

Best,

-Nate