Re: CREATE TABLE .. LIKE copies comments to an unrelated table - Mailing list pgsql-hackers

On Thu, Oct 1, 2026 at 10:35 PM Jim Jones <jim.jones@uni-muenster.de> wrote:
>
> Hi
>
> While working on another patch I found that CREATE TABLE ... LIKE
> INCLUDING COMMENTS copies the comments to the wrong table when the
> target is a temp table and a table with the same name exists in a schema
> listed before pg_temp in search_path.
>
> Example:
>
> psql (18.3 (Debian 18.3-1.pgdg13+1))
> Type "help" for help.
>
> db=> CREATE TABLE src (a int);
> CREATE TABLE
> db=> COMMENT ON COLUMN src.a IS 'col comment';
> COMMENT
> db=> CREATE TABLE foo (a int); -- unrelated table
> CREATE TABLE
> db=> COMMENT ON COLUMN foo.a IS 'important comment';
> COMMENT
> db=> SET search_path = public, pg_temp;
> SET
> db=> CREATE TEMP TABLE foo (LIKE src INCLUDING COMMENTS);
> CREATE TABLE
> db=> SELECT n.nspname, relname, col_description(c.oid, 1) AS colcomment
> FROM pg_class c JOIN pg_namespace n ON n.oid = c.relnamespace
> WHERE c.relname = 'foo';
>   nspname   | relname | colcomment
> ------------+---------+-------------
>  public     | foo     | col comment
>  pg_temp_24 | foo     |
> (2 rows)
>
> The unrelated public.foo got the column comment and the created
> temporary table got none -- public.foo.a comment was overwritten. Is
> this the expected behaviour?

I think this is a bug, and I can reproduce it on master.

>
> Setting the schema to pg_temp in case of RELPERSISTENCE_TEMP in
> transformCreateStmt seems to do the trick (but I didn't dive too deep in
> the code just yet):
>
> -       if (stmt->relation->schemaname == NULL
> -               && stmt->relation->relpersistence != RELPERSISTENCE_TEMP)
> -               stmt->relation->schemaname =
> get_namespace_name(namespaceid);
> +       if (stmt->relation->schemaname == NULL)
> +       {
> +               if (stmt->relation->relpersistence == RELPERSISTENCE_TEMP)
> +                       stmt->relation->schemaname = pstrdup("pg_temp");
> +               else
> +                       stmt->relation->schemaname =
> get_namespace_name(namespaceid);
> +       }

The preceding RangeVarGetAndCheckCreationNamespace has already
initialized the temporary namespace and returned its OID, so simplified it
to:

if (stmt->relation->schemaname == NULL)
    stmt->relation->schemaname = get_namespace_name(namespaceid);

should work, I did a quick test to verify this.

>
> With my changes in HEAD:
>
> db=> CREATE TABLE src (a int);
> CREATE TABLE
> db=> COMMENT ON COLUMN src.a IS 'col comment';
> COMMENT
> db=> CREATE TABLE foo (a int); -- unrelated table
> CREATE TABLE
> db=> COMMENT ON COLUMN foo.a IS 'important comment';
> COMMENT
> db=> SET search_path = public, pg_temp;
> SET
> db=> CREATE TEMP TABLE foo (LIKE src INCLUDING COMMENTS);
> CREATE TABLE
> db=> SELECT n.nspname, relname, col_description(c.oid, 1) AS colcomment
> FROM pg_class c JOIN pg_namespace n ON n.oid = c.relnamespace
> WHERE c.relname = 'foo';
>   nspname   | relname |    colcomment
> ------------+---------+-------------------
>  public     | foo     | important comment
>  pg_temp_92 | foo     | col comment
> (2 rows)
>
>
> WDYT?
>
> Best, Jim
>
>
>


--
Regards
Junwang Zhao