Bug #19030 [1] seems to be a fresh report of the problem this patch
aims to solve. While answering that, I realized that the v2 patch
causes null-keyed inner rows to not be included in EXPLAIN ANALYZE's
report of the number of rows output by the Hash node. Now on the
one hand, what it's reporting is an accurate reflection of the
number of rows in the hash table, which perhaps is useful. On the
other hand, it's almost surely going to confuse users, and it's
different from the number we produced before. Should we try to
preserve the old behavior here? (I've not looked at what code
changes would be needed for that.)
regards, tom lane
[1] https://www.postgresql.org/message-id/flat/19030-944dd78d7ef94c0f%40postgresql.org