Re: [PATCH] Fix ALTER SYSTEM empty string bug for GUC_LIST_QUOTE parameters

Hi Tom

On 05.09.25 23:06, Tom Lane wrote:
> I remain unsure which way I like better. The NULL approach has the
> advantage of not foreclosing use of empty-string list elements, which
> we might want someday even if there's no obvious value today. (And
> for the same reason, it's less of a behavioral change.) But it still
> feels a bit less intuitive to me.

I think this is a nice addition. The way I see it is: it provides an
unambiguous way to "clear" the variable, which, as you pointed out,
might carry different semantics in the future than an empty string. More
generally, I understand that using NULL (unknown/undefined) to represent
an empty list could be seen as a semantic stretch, but in this case it
doesn’t feel unintuitive to me. Although I prefer this new syntax, I can
definitely live without it :)

Here some tests:

== ALTER SYSTEM SET var TO ''

$ 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 = '""'

$ pg_ctl -D /usr/local/postgres-dev/testdb -l
/usr/local/postgres-dev/logfile restart
waiting for server to shut down.... done
server stopped
waiting for server to start.... done
server started

$ psql postgres
psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed:
FATAL:  could not access file "$libdir/plugins/": No such file or directory

The error itself is expected, but the message does not make it
immediately clear that the problem comes from a misconfigured GUC. But
this seems to be more related to the specific variable than to the scope
of this patch.

== ALTER SYSTEM SET var TO NULL
Using the new syntax it works just fine:

$ psql postgres -c "ALTER SYSTEM SET local_preload_libraries TO NULL;"
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 = ''

$ pg_ctl -D /usr/local/postgres-dev/testdb -l
/usr/local/postgres-dev/logfile restart
waiting for server to shut down.... done
server stopped
waiting for server to start.... done
server started

$ psql postgres -c "SHOW local_preload_libraries;"
 local_preload_libraries
-------------------------
 
(1 row)

== SET var TO ''

$ psql postgres -c "SET local_preload_libraries TO ''; SHOW
local_preload_libraries;"
SET
 local_preload_libraries
-------------------------
 ""
(1 row)

== SET var TO NULL

$ psql postgres -c "SET local_preload_libraries TO NULL; SHOW
local_preload_libraries;"
SET
 local_preload_libraries
-------------------------
 
(1 row)

== SET var TO list containing empty element

$ psql postgres -c "SET local_preload_libraries TO 'foo',''; SHOW
local_preload_libraries;"
SET
 local_preload_libraries
-------------------------
 foo, ""
(1 row)

== SET var TO list containing NULL element

$ psql postgres -c "SET local_preload_libraries TO NULL,''; SHOW
local_preload_libraries;"
ERROR:  syntax error at or near ","
LINE 1: SET local_preload_libraries TO NULL,''; SHOW local_preload_l...

== SET var TO list containing multiple empty elements

$ /usr/local/postgres-dev/bin/psql postgres -c "SET
local_preload_libraries TO '',''; SHOW local_preload_libraries;"
SET
 local_preload_libraries
-------------------------
 "", ""
(1 row)

Best regards, Jim