BUG #18217: ALTER TABLE IF EXISTS not being honored with DROP CONSTRAINT IF EXISTS

The following bug has been logged on the website:

Bug reference:      18217
Logged by:          David Moss
Email address:      david@buildingfootprintusa.com
PostgreSQL version: 15.2
Operating system:   PostgreSQL 15.2 on x86_64-pc-linux-gnu
Description:

I found this issue from running pg_restore with flags to clean and "if
exists".  pg_restore returns an error code which is generated from a query
of the following form:

ALTER TABLE IF EXISTS ONLY public.foo DROP CONSTRAINT IF EXISTS foo_pkey

In pg_admin this reports:

ERROR:  relation "public.foo" does not exist 
SQL state: 42P01

Running this same query in a 15.1 instance returns the expected behavior:

NOTICE:  relation "foo" does not exist, skipping
ALTER TABLE
Query returned successfully in 48 msec.

More detailed version info:

PostgreSQL 15.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 7.3.1 20180712
(Red Hat 7.3.1-12), 64-bit
vs.
PostgreSQL 15.1, compiled by Visual C++ build 1914, 64-bit