Обсуждение: Inconsistencies around Composite Row nullness
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)
This next statement resolves to ROW(NULL, 2):
SELECT coalesce(ROW(NULL::integer, 2), ROW(1, 2))
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)
Thank you!
Chris
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-bitI 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-bitThis 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.
Chris Hanks <christopher.m.hanks@gmail.com> writes:
> 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
Yeah, it's not terribly consistent, but neither is the SQL standard
in this area. I believe what is happening in your first example
is that the construct "ROW(a, b, ...) = ROW(x, y, ...)" is being
broken down into "(a = x) AND (b = y) AND ...", from which you can
get a NULL result as described. However, the insertion of coalesce()
stops that decomposition from happening, and then what you get is the
behavior of the native composite-type comparators (record_eq and
friends). Those functions adhere to the btree requirement of
producing a total order of the datatype, ie null results are not OK,
so they report that ROW(NULL, 2) = ROW(NULL, 2) is true not null.
We could get rid of some of the inconsistency by eliminating that
special treatment of equality of two row-constructors, but I'm
afraid there would be complaints from people who were relying on
that behavior for optimization purposes.
regards, tom lane