set-valued function called in context that cannot accept a set

["Eric B. Ridge" <ebr@xxxxxxxx>]

This is using PG v8.1.

I have a "table function" in C called "unnest".  It takes "anyarray"  
as its only argument and returns a set of "anyelement".  It's a handy  
little function for turning arrays into sets.

You can use it in two different ways:
	SELECT * FROM unnest(ARRAY[1,2,3]);
or
	SELECT unnest(ARRAY[1,2,3]);

The latter is particularly handy when used like this:

# select unnest(ARRAY[1,2,3]), 'hi';
 unnest | ?column?
--------+----------
      1 | hi
      2 | hi
      3 | hi
(3 rows)


I decided that this function would be easy to rewrite in PL/PGSQL and  
then I could stop compiling an extra library every time I install  
Postgres.

CREATE OR REPLACE FUNCTION unnest2 (_a anyarray) RETURNS SETOF  
anyelement LANGUAGE plpgsql IMMUTABLE AS $$
DECLARE
	i int;
	upper int;
BEGIN
	i := 0;
	upper := array_upper(_a, 1);
	FOR i IN 1..upper LOOP
		RETURN NEXT _a[i];
	END LOOP;
	RETURN;
END;
$$;

However, the PL/PGSQL version cannot be used as a column.  Doing so  
does results in an error:

# select unnest2(ARRAY[1,2,3]), 'hi';
ERROR:  set-valued function called in context that cannot accept a set
CONTEXT:  PL/pgSQL function "unnest2" line 8 at return next

Is the definition of "unnest2" wrong or is this just a limitation of  
PL/PGSQL?  If this can't be done via PL/PGSQL in v8.1, what about v8.3  
(or later)?

Any input will be greatly appreciated!

eric


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