Thread: Terrible plan for join to nested union

Nate Allan <nallan@ancestry.com> writes:
> 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)

What exactly are you trying to accomplish here?  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.

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

select 'anything' as result
               from "Attribute" as A1
               where (A1."ID" = 124791200)
                  and exists (
                              select 1 from "Relationship" as R
                                where R."TargetID" = A1."PersonID"
                              union all
                              select 1 from "Attribute" as A2
                                where A2."PersonID" = A1."PersonID"
                             )

since you're evidently hoping that the EXISTS won't need to be evaluated
for very many rows of A1.  Or you could use an OR of two EXISTS to skip
the UNION altogether.

            regards, tom lane

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

2012/7/8 Nate Allan <nallan@ancestry.com>:
> 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
decentperformance-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
aMerge Join. 
>
>>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
fora work-around but a) wondering why the plan is so bad; and b) asking if it could be fixed if possible.
Unfortunatelyrewriting the query isn't a trivial matter in our case because the X (union) part of the query is
representedlogically as a view, which is expected to be restricted and/or joined so as not to actually materialize the
actualunion.  Unfortunately the PostgreSQL planner seems to want to actually materialize that view.  Working around
thiswould basically entail not using the view, which is used all over the place, and instead duplicating the view's
logicexcept pushing the restrictions and/or joins down into both sides of the union in each case.  I could do that, but
doingso would be: a) against the spirit of the Relational Model; b) against the spirit of "fix the planner rather than
addoptimizer hints"; c) a royal pain because it causes a rewrite of application logic; d) a point for at least one
otherDBMS's optimizer.  :-) 

you are using EAV schema - it is against to relation model enough :)

this schema has the most terrible performance for large datasets -
looks on hstore instead

Regards

Pavel

>
>>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
>
>
>
> --
> Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
> To make changes to your subscription:
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>you are using EAV schema - it is against to relation model enough :)
>this schema has the most terrible performance for large datasets - looks on hstore instead

>Pavel

Actually despite the table named Attribute, I am not doing EAV though I can see why you'd think that.  Attributes are
partof the conceptual domain I'm modeling and I assure you there are first class columns in the schema for everything.
Regardless,that has nothing to do with my performance problem with joining to a nested union.
 

-Nate

Nate Allan <nallan@ancestry.com> writes:
> 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

[ eyes that suspiciously ... ]  I'm not convinced that such a
transformation is either correct in general (you seem to be assuming
at least that A's join column is unique, and what is the UNION operator
supposed to do with A's other columns?) or likely to lead to a
performance improvement in general.

We possibly could push down a join condition on the inner side of a
nestloop, similarly to what's done in the UNION ALL case ... but that
would require a complete refactoring of what the planner does with
UNIONs.  By and large, very little optimization effort has been put
into non-ALL UNION (or INTERSECT or EXCEPT).  You should not expect
that to change on a time scale of less than years.

            regards, tom lane

>>Nate Allan <nallan@ancestry.com> writes:
>> It seems to me that the join condition (and hence the restriction) should be pushed down into both sides of the
unionto bring 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

>[ eyes that suspiciously ... ]  I'm not convinced that such a transformation is either correct in general (you seem to
beassuming at least that A's join column is unique, and >what is the UNION operator supposed to do with A's other
columns?)or likely to lead to a performance improvement in general. 

If there are more columns, you are correct that you might have to project off any additional columns within the union,
andleave the join outside of the union intact to bring in the extra columns.  Those are essentially the same
considerationsas when making other rewrites though.  As for this optimization making unions faster in general, I would
arguethat it is rather easy to produce a plan superior to complete materialization of the union. 

>We possibly could push down a join condition on the inner side of a nestloop, similarly to what's done in the UNION
ALLcase ... but that would require a complete >refactoring of what the planner does with UNIONs.  By and large, very
littleoptimization effort has been put into non-ALL UNION (or INTERSECT or EXCEPT).  You should >not expect that to
changeon a time scale of less than years. 

I hate to come across as contrary, but I'm pretty shocked by this answer for a couple reasons:
1) This is a clear-cut case of an untenable execution plan, essentially a bug in the planner.  This response
contradictsthe widely broadcast assertion that the PG community fixes planner bugs quickly and will not introduce hints
becausethey would rather address these kinds of issues "correctly". 
2) Why would more effort go into Union All rather than Union?  Are people using Union All more than Union, and if so is
thisbecause they actually want duplicates or is it because they've been trained to due to the performance problems with
Union? Union All, in many people's opinions, shouldn't even exist in a true relational sense. 

Again, sorry if I'm coming off as abrasive, I've spent political capital pushing to get PG in on this project, and now
I'ma little worried about whether it is going to work for this kind of scale and complexity, so I'm a little stressed.
Ido appreciate your responses. 

Best,

-Nate

Nate Allan <nallan@ancestry.com> writes:
> 2) Why would more effort go into Union All rather than Union?

The UNION ALL case matches up with, and shares planning and execution
code with, table-inheritance and partitioning scenarios.  So yes, it
really is more interesting to more people than UNION DISTINCT.
(IIRC, the code that does that stuff was originally meant to support the
inheritance case, and we hacked UNION ALL to be able to share the logic,
not vice versa.)

Right now, UNION DISTINCT, along with INTERSECT and EXCEPT, have
basically no optimization support whatsoever: all of them go through a
code path that just evaluates both input relations and performs the
set-combination operation.  All of that code dates from a period about
a dozen years ago when we were more interested in getting the right
answer at all than how fast it was.  Rewriting it all to have some
optimization capability is certainly on the wish-list ... but the fact
that it hasn't risen to the top of anybody's to-do list in that time
indicates to me that it probably isn't going to get done in the next
little while either.  And even if someone were to start working on it
right now, it's not a small project.

Sorry to be the bearer of bad news, but this isn't going to change
just because you try to label it a bug.

            regards, tom lane

>Right now, UNION DISTINCT, along with INTERSECT and EXCEPT, have basically no optimization support whatsoever...
> Sorry to be the bearer of bad news, but this isn't going to change just because you try to label it a bug.

Given the medium, I'll try not to read that in a snarky tone, after all, surely it's not unreasonable to label it a
defectfor a system not to optimize one of the basic relational primitives.  That said, I know well the annoyance when a
usercries bug when the system is working as-designed.  In any case, I'm at least glad to have resolution; I know that
thereis no choice but to work around it. 

For a maximally general work-around given that the union is the essence of a reused view, perhaps a reasonable approach
isto switch to Union All and nest it within a Distinct outer query.  That seems to produce workable plans in my tests
sofar.  Maybe that could even form the basis of a planner enhancement that wouldn't require a complete refactor. 

Thanks again,

-Nate