On 04.09.25 23:52, Tom Lane wrote: > Note that the patch includes changing SplitIdentifierString and its > clones to forbid zero-length quoted elements, which were formerly > allowed. Without this, we'd accept values from config files that > could not be represented in SET, which is exactly the situation we > are trying to fix. > > I'm not entirely sure if this is the way to go, or if we want to > adopt some other solution that doesn't involve forbidding empty > list elements. I suspect that anything else we come up with would > be less intuitive than letting SET list_var = '' do the job; > but maybe I just lack imagination today.
This approach LGTM. It solves the initial issue:
$ psql postgres -c "ALTER SYSTEM SET local_preload_libraries TO '';" ALTER SYSTEM
$ cat /usr/local/postgres-dev/testdb/postgresql.auto.conf # Do not edit this file manually! # It will be overwritten by the ALTER SYSTEM command. local_preload_libraries = ''
... making a clear distinction between empty elements and empty lists:
postgres=# SET local_preload_libraries TO '','foo'; ERROR: SET local_preload_libraries does not accept empty-string elements
postgres=# SET local_preload_libraries TO ''; SET postgres=# SHOW local_preload_libraries; local_preload_libraries -------------------------
(1 row)
The ambiguity between an empty list and an empty element has always existed in list-valued GUCs. This patch resolves the issue by disallowing empty elements, thereby making '' an unambiguous representation of an empty list. Personally, I find SET var TO NULL (or perhaps a keyword like EMPTY or NONE) a more palatable syntax for expressing empty lists in this case. However, I’m not sure the additional complexity and compatibility implications would justify such a change.
