Which side of a Merge Join gets executed first? Do both sides always get executed?

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

[image: image.png]

NOTE:

- The query plan in 13 is slightly different, but still includes the
Merge Joins.
- Replacing ANY(ARRAY(<subquery)) with IN(<subquery>) fixes the
performance problem, but we'd still like to understand the execution
characteristics of Merge Join

Thanks,
Jerry