This example is lacking indexes on the child tables, which is
why the plan shown is about as good as you're going to get.
The contents of foo1 and foo2 have to be read in entirety in any
case, and sorting them separately is not a win compared to doing
a single sort.
It is true, but not in case of FDW connected to remote host.
In this case sending large volumes of data through network will be very inefficient.

There will be no problem if FDW can provide index scan - in this case MergeAppend will fetch only required number of records:

postgres=# explain analyze select * from t order by u limit 1;
                                                      QUERY PLAN
Limit (cost=300.17..300.23 rows=1 width=8) (actual time=4.588..4.588 rows=1 loops=1) -> Merge Append (cost=300.17..762.76 rows=7681 width=8) (actual time=4.586..4.586 rows=1 loops=1)
         Sort Key: t.u
-> Index Scan using t_pkey on t (cost=0.12..8.14 rows=1 width=8) (actual time=0.003..0.003 rows=0 loops=1) -> Foreign Scan on t_fdw1 (cost=100.00..193.92 rows=2560 width=8) (actual time=1.532..1.532 rows=1 loops=1) -> Foreign Scan on t_fdw2 (cost=100.00..193.92 rows=2560 width=8) (actual time=1.510..1.510 rows=1 loops=1) -> Foreign Scan on t_fdw3 (cost=100.00..193.92 rows=2560 width=8) (actual time=1.535..1.535 rows=1 loops=1)

But if sort is performed by non-indexed fields, then current behaviour will be inefficient and can be significantly improved by pushing limits to remote hosts.

Konstantin Knizhnik
Postgres Professional:
The Russian Postgres Company

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