The following is also pretty strange. Why should adding the SRF column to the ORDER BY change the number of output rows?
postgres=# select distinct on (a) a,b, generate_Series(1,2) from ab order by a,b,3 desc; a | b | generate_series ---+---+----------------- 1 | 1 | 2 2 | 1 | 2 (2 rows)
postgres=# select distinct on (a) a,b, generate_Series(1,2) from ab order by a,b; a | b | generate_series ---+---+----------------- 1 | 1 | 1 1 | 1 | 2 2 | 1 | 1 2 | 1 | 2 (4 rows)
This is indeed weird. This seems depends on at which plan level we add ProjectSetPath. For the first query we insert ProjectSetPath at the scan/join level, because the SRF is included in the ORDER BY, which makes the SRF appear in the scan/join target. For the second query the SRF is postponed to after the Sort.
