On Sunday, November 2, 2025, Chris Hanks <
christopher.m.hanks@gmail.com> wrote:
Hello -
I've experienced some logically inconsistent query output on my local Postgres instance, version string: PostgreSQL 18.0 (Homebrew) on aarch64-apple-darwin25.0.0, compiled by Apple clang version 17.0.0 (clang-1700.3.19.1), 64-bit
I also reproduced it on the most recent Postgres version available at
db-fiddle.com, version string: PostgreSQL 17.0 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 11.4.1 20230605 (Red Hat 11.4.1-2), 64-bit
This first statement resolves, reasonably, to NULL:
SELECT ROW(NULL::integer, 2) = ROW(NULL::integer, 2)
Yes, ROW constructed values within an equality resolve using SQL row constructor comparison rules.
This next statement resolves to ROW(NULL, 2):
SELECT coalesce(ROW(NULL::integer, 2), ROW(1, 2))
This statement is technically impossible - nothing resolves to “ROW(…)” - the fact that ROW (a row constructor) is involved is erased when passing the result of the expression through a function such that a plain composite/record is produced. It is necessary, for the rest of the system to function correctly, that records are comparable using (null equals null => true) semantics (is distinct; composite type comparison).
These final two statements each resolve to true, which is inconsistent with the previous statements (each should resolve to NULL):
SELECT coalesce(ROW(NULL::integer, 2), ROW(1, 2)) = ROW(NULL::integer, 2)
SELECT coalesce(ROW(NULL::integer, 2)) = ROW(NULL::integer, 2)
See specifically the commentary in row constructor comparison 9.25.5 and composite type comparison 9.25.6 in the documentation.
There is a patch to further expound/consolidate discussion on this topic (null handling in PostgreSQL) presently awaiting committer attention.
David J.
