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


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