Re: Planner not using column limit specified for one column for another column equal to first

Віталій Тимчишин wrote:

Hello.

I have a query that performs very poor because there is a limit on join column that is not applied to other columns:

select * from company this_ left outer join company_tag this_1_ on this_.id=this_1_.company_id left outer join company_measures companymea2_ on this_.id=companymea2_.company_id left outer join company_descr ces3_ on this_.id=ces3_.company_id where this_1_.tag_id = 7 and this_.id>50000000 and this_1_.company_id>50000000
order by this_.id asc limit 1000;

(plan1.txt)
Total runtime: 7794.692 ms

At the same time if I apply the limit (>50000000) to other columns in query itself it works like a charm:

select * from company this_ left outer join company_tag this_1_ on this_.id=this_1_.company_id left outer join company_measures companymea2_ on this_.id=companymea2_.company_id left outer join company_descr ces3_ on this_.id=ces3_.company_id where this_1_.tag_id = 7 and this_.id>50000000 and this_1_.company_id>50000000
and companymea2_.company_id>50000000 and ces3_.company_id>50000000
order by this_.id asc limit 1000;

(plan2.txt)
Total runtime: 27.547 ms

I've thought and someone in this list've told me that this should be done automatically.

Yes, if you have in a query a=b and b=c, then the optimizer figures out that a=c as well. (a,b and c are then member of the same equivalence class).

However both queries are not the same, since the joins you're using are outer joins. In the first it's possible that records are returned for company records with no matching ces3_ records, the ces3_ records is null in that case. In the second query no NULL ces3_ information may be returned.

Another thing is it seems that the number of rows guessed is far off from the actual number of rows, is the number 5000000 artificial or are you're statistics old or too small histogram/mcv's?