Le 20/12/2023 à 15:40, Jerry Brenner a écrit :
The attached query plan is from 11.
We are getting Merge Joins on both sides of the UNION. In both cases,
the first node under the Merge Join returns 0 rows but the other side of
the Merge Join (the one being sorted) is executed and that's where all
of the time is spent.
On the surface, I don't see any way from the attached explain plan to
determine which side of the Merge Join is executed first. Some questions:
* Which side gets executed first?
* How would one tell that from the json?
* Have there been any relevant changes to later releases to make that
more apparent?
* Whichever side gets executed first, is the execution of the side
that would be second get short circuited if 0 rows are returned by
the first side?
Here's a screenshot from pgMustard.
* Nodes 6 and 14 (the first node under each of the Merge Joins) each
return 0 rows
* Nodes 9 and 15 are the expensive sides of the Merge Joins and return
lots of rows
I think those nodes (9 and 15) are expensive because they have to filter
out 8 millions rows in order to produce their first output row. After
that, they get short circuited.
Best regards,
Frédéric
