Allow "SET list_guc TO NULL" to specify setting the GUC to empty.

We have never had a SET syntax that allows setting a GUC_LIST_INPUT
parameter to be an empty list.  A locution such as
	SET search_path = '';
doesn't mean that; it means setting the GUC to contain a single item
that is an empty string.  (For search_path the net effect is much the
same, because search_path ignores invalid schema names and '' must be
invalid.)  This is confusing, not least because configuration-file
entries and the set_config() function can easily produce empty-list
values.

We considered making the empty-string syntax do this, but that would
foreclose ever allowing empty-string items to be valid in list GUCs.
While there isn't any obvious use-case for that today, it feels like
the kind of restriction that might hurt someday.  Instead, let's
accept the forbidden-up-to-now value NULL and treat that as meaning an
empty list.  (An objection to this could be "what if we someday want
to allow NULL as a GUC value?".  That seems unlikely though, and even
if we did allow it for scalar GUCs, we could continue to treat it as
meaning an empty list for list GUCs.)

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrei Klychkov <andrew.a.klychkov@gmail.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/CA+mfrmwsBmYsJayWjc8bJmicxc3phZcHHY=yW5aYe=P-1d_4bg@mail.gmail.com

1 files changed, 11 insertions, 1 deletions

diff --git a/doc/src/sgml/ref/alter_system.sgml b/doc/src/sgml/ref/alter_system.sgml
index 1bde66d6ad2..b28919d1b26 100644
--- a/doc/src/sgml/ref/alter_system.sgml
+++ b/doc/src/sgml/ref/alter_system.sgml
@@ -84,6 +84,8 @@ ALTER SYSTEM RESET ALL
       constants, identifiers, numbers, or comma-separated lists of
       these, as appropriate for the particular parameter.
       Values that are neither numbers nor valid identifiers must be quoted.
+      If the parameter accepts a list of values, <literal>NULL</literal>
+      can be written to specify an empty list.
       <literal>DEFAULT</literal> can be written to specify removing the
       parameter and its value from <filename>postgresql.auto.conf</filename>.
      </para>
@@ -136,7 +138,15 @@ ALTER SYSTEM SET wal_level = replica;
    in <filename>postgresql.conf</filename>:
 <programlisting>
 ALTER SYSTEM RESET wal_level;
-</programlisting></para>
+</programlisting>
+  </para>
+
+  <para>
+   Set the list of preloaded extension modules to be empty:
+<programlisting>
+ALTER SYSTEM SET shared_preload_libraries TO NULL;
+</programlisting>
+  </para>
  </refsect1>
 
  <refsect1>