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

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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
This commit is contained in:
Tom Lane
2025-11-04 12:37:40 -05:00
parent 93b7ab5b4b
commit ff4597acd4
12 changed files with 135 additions and 17 deletions
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12
doc/src/sgml/ref/alter_system.sgml
View File

@@ -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>

24
doc/src/sgml/ref/set.sgml
View File

@@ -21,8 +21,8 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
SET [ SESSION | LOCAL ] <replaceable class="parameter">configuration_parameter</replaceable> { TO | = } { <replaceable class="parameter">value</replaceable> | '<replaceable class="parameter">value</replaceable>' | DEFAULT }
SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replaceable> | '<replaceable class="parameter">value</replaceable>' | LOCAL | DEFAULT }
SET [ SESSION | LOCAL ] <replaceable class="parameter">configuration_parameter</replaceable> { TO | = } { <replaceable class="parameter">value</replaceable> [, ...] | DEFAULT }
SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replaceable> | LOCAL | DEFAULT }
</synopsis>
</refsynopsisdiv>
@@ -123,7 +123,7 @@ SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replac
<term><replaceable class="parameter">configuration_parameter</replaceable></term>
<listitem>
<para>
Name of a settable run-time parameter. Available parameters are
Name of a settable configuration parameter. Available parameters are
documented in <xref linkend="runtime-config"/> and below.
</para>
</listitem>
@@ -133,9 +133,12 @@ SET [ SESSION | LOCAL ] TIME ZONE { <replaceable class="parameter">value</replac
<term><replaceable class="parameter">value</replaceable></term>
<listitem>
<para>
New value of parameter. Values can be specified as string
New value of the parameter. Values can be specified as string
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
resetting the parameter to its default value (that is, whatever
value it would have had if no <command>SET</command> had been executed
@@ -283,6 +286,19 @@ SELECT setseed(<replaceable>value</replaceable>);
Set the schema search path:
<programlisting>
SET search_path TO my_schema, public;
</programlisting>
Note that this is not the same as
<programlisting>
SET search_path TO 'my_schema, public';
</programlisting>
which would have the effect of setting <varname>search_path</varname>
to contain a single, probably-nonexistent schema name.
</para>
<para>
Set the list of temporary tablespace names to be empty:
<programlisting>
SET temp_tablespaces TO NULL;
</programlisting>
</para>