While working on a bug in postgres_fdw, I found this:
NestLoop
-> Seq Scan on SmallTable1 A
NestLoop
-> Seq Scan on SmallTable2 B
NestLoop
-> Index Scan using XYIndex on LargeTable C
Index Condition: C.X = A.AID and C.Y = B.BID
This seems odd to me; we would not need the bottom-level Nestloop.
Attached is a small patch for fixing that. I also adjusted the
indentation to the last line "Index Condition: ...", to match others in
the section of Parameterized Paths.
Best regards,
Etsuro Fujita
*** a/src/backend/optimizer/README
--- b/src/backend/optimizer/README
***************
*** 756,764 **** to create a plan like
-> Seq Scan on SmallTable1 A
NestLoop
-> Seq Scan on SmallTable2 B
! NestLoop
! -> Index Scan using XYIndex on LargeTable C
! Index Condition: C.X = A.AID and C.Y = B.BID
so we should be willing to pass down A.AID through a join even though
there is no join order constraint forcing the plan to look like this.
--- 756,763 ----
-> Seq Scan on SmallTable1 A
NestLoop
-> Seq Scan on SmallTable2 B
! -> Index Scan using XYIndex on LargeTable C
! Index Condition: C.X = A.AID and C.Y = B.BID
so we should be willing to pass down A.AID through a join even though
there is no join order constraint forcing the plan to look like this.
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