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doc: Clarify that empty COMMENT string removes the comment.

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Clarify the documentation of COMMENT ON to state that specifying an empty
string is treated as NULL, meaning that the comment is removed.

This makes the behavior explicit and avoids possible confusion about how
empty strings are handled.

Also adds regress test cases that use empty string to remove a comment.

Backpatch to all supported versions.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Shengbin Zhao <zshengbin91@gmail.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: zhangqiang <zhang_qiang81@163.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/26476097-B1C1-4BA8-AA92-0AD0B8EC7190@gmail.com
Backpatch-through: 14
This commit is contained in:
Fujii Masao
2026-03-03 14:47:41 +09:00
parent 858c83b3e6
commit 7ab6ac1d7f
3 changed files with 29 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
doc/src/sgml/ref/comment.sgml
View File

@ -80,14 +80,16 @@ COMMENT ON
<title>Description</title>
<para>
<command>COMMENT</command> stores a comment about a database object.
<command>COMMENT</command> stores, replaces, or removes the comment on a
database object.
</para>
<para>
Only one comment string is stored for each object, so to modify a comment,
issue a new <command>COMMENT</command> command for the same object. To remove a
comment, write <literal>NULL</literal> in place of the text string.
Comments are automatically dropped when their object is dropped.
Only one comment string is stored for each object. Issuing a new
<command>COMMENT</command> command for the same object replaces the
existing comment. Specifying <literal>NULL</literal> or an empty
string (<literal>''</literal>) removes the comment. Comments are
automatically dropped when their object is dropped.
</para>
<para>
@ -265,7 +267,8 @@ COMMENT ON
<term><replaceable class="parameter">string_literal</replaceable></term>
<listitem>
<para>
The new comment contents, written as a string literal.
The new comment contents, written as a string literal. An empty string
(<literal>''</literal>) removes the comment.
</para>
</listitem>
</varlistentry>
@ -274,7 +277,7 @@ COMMENT ON
<term><literal>NULL</literal></term>
<listitem>
<para>
Write <literal>NULL</literal> to drop the comment.
Write <literal>NULL</literal> to remove the comment.
</para>
</listitem>
</varlistentry>
@ -361,6 +364,7 @@ COMMENT ON TRANSFORM FOR hstore LANGUAGE plpythonu IS 'Transform between hstore
COMMENT ON TRIGGER my_trigger ON my_table IS 'Used for RI';
COMMENT ON TYPE complex IS 'Complex number data type';
COMMENT ON VIEW my_view IS 'View of departmental costs';
COMMENT ON VIEW my_view IS NULL;
</programlisting></para>
</refsect1>

14
src/test/regress/expected/create_index.out
View File

@ -30,6 +30,20 @@ COMMENT ON INDEX six_wrong IS 'bad index';
ERROR: relation "six_wrong" does not exist
COMMENT ON INDEX six IS 'good index';
COMMENT ON INDEX six IS NULL;
SELECT obj_description('six'::regclass, 'pg_class') IS NULL AS six_comment_is_null;
six_comment_is_null
---------------------
t
(1 row)
COMMENT ON INDEX six IS 'add the comment back';
COMMENT ON INDEX six IS ''; -- empty string removes the comment, same as NULL
SELECT obj_description('six'::regclass, 'pg_class') IS NULL AS six_comment_is_null;
six_comment_is_null
---------------------
t
(1 row)
--
-- BTREE ascending/descending cases
--

4
src/test/regress/sql/create_index.sql
View File

@ -42,6 +42,10 @@ CREATE INDEX six ON shighway USING btree (name text_ops);
COMMENT ON INDEX six_wrong IS 'bad index';
COMMENT ON INDEX six IS 'good index';
COMMENT ON INDEX six IS NULL;
SELECT obj_description('six'::regclass, 'pg_class') IS NULL AS six_comment_is_null;
COMMENT ON INDEX six IS 'add the comment back';
COMMENT ON INDEX six IS ''; -- empty string removes the comment, same as NULL
SELECT obj_description('six'::regclass, 'pg_class') IS NULL AS six_comment_is_null;
--
-- BTREE ascending/descending cases