Re: Asynchronous MergeAppend

Hi,

I reviewed and tested the latest v22 patch series for Asynchronous MergeAppend.
I was able to reproduce the reported behavior using two postgres_fdw
servers with async_capable = on.
Before applying the patch, I ran the following query

EXPLAIN (VERBOSE, COSTS OFF)
SELECT *
FROM players_remote1
UNION ALL
SELECT *
FROM players_remote2
ORDER BY score, id;

The plan showed
Merge Append
-> Foreign Scan on public.players_remote1
-> Foreign Scan on public.players_remote2

So even though both foreign servers were configured with async_capable
= on, the MergeAppend plan was still using regular Foreign Scan nodes.
I then applied the v22 patch series, rebuilt PostgreSQL and ran the
same test again.

After applying the patch, the plan changed to
Merge Append
-> Async Foreign Scan on public.players_remote1
-> Async Foreign Scan on public.players_remote2

This looks correct to me and shows that MergeAppend is now able to
make use of the asynchronous foreign scan capability.
I also checked the query result after the patch.
The two remote tables contained 5,000 rows each and the query
returned all 10,000 rows as expected.

I did not run a performance comparison, so I have not evaluated the
actual performance improvement.
Overall, the patch worked as expected in my testing.

Regards,
solai