The following bug has been logged online:

Bug reference: 1702
Logged by: Jan Behrens
Email address: jan (AT) monso (DOT) de
PostgreSQL version: 8.0.3
Operating system: NetBSD
Description: Function returning nested composite types
Details:

Following input:

CREATE TYPE test1 AS (a text, b text);
CREATE TYPE test2 AS (c test1, d test1);

CREATE FUNCTION test3 () RETURNS test2
LANGUAGE 'plpgsql' AS $$
DECLARE
result test2;
BEGIN
result := ROW (ROW ('1', '2'), ROW ('3', '4'));
RETURN result;
END; $$;

SELECT * FROM test3();


generates the following error:

ERROR: row not compatible with its own tupdesc
CONTEXT: PL/pgSQL function "test3" line 5 at return


I assume this is a bug, because the source code says:

if (estate->retval == (Datum) NULL)
/* should not happen */
elog(ERROR,
"row not compatible with its own tupdesc");
[...]
if (tup == NULL) /* should not happen */
elog(ERROR,
"row not compatible with its own tupdesc");

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