Composite data type missing value when returned from function

Question

I have a function returning a setof a composite data type. When I run the function, only one of the two columns defined in the composite data type has data in it. When troubleshooting the problem I did a RAISE NOTICE on the composite data type and it has data in the column that is missing data in the result set. I tested both before and after the RETURN NEXT. I am using PostgreSQL 9.2.
Any idea why the result set is missing the data?

CREATE TYPE map_data AS (lid uuid, tile_ids uuid [] );

CREATE OR REPLACE FUNCTION get_map_data(mid_val uuid) RETURNS SETOF map_data AS $$
DECLARE
    j uuid;
    b uuid;
    m map_data%rowtype;
    tid uuid []; --tile ids
BEGIN
FOR j IN
    SELECT lid FROM map_layers WHERE "mid" = mid_val
LOOP
    FOREACH b IN ARRAY (SELECT tiles FROM layer_tiles WHERE lid = j) 
    LOOP
        tid := array_append(tid,b);
    END LOOP;
    SELECT j, tid INTO m;
    RETURN NEXT m;
    tid := '{}'; --clear the array.
END LOOP;
RETURN;
END;
$$ LANGUAGE plpgsql;

Answer 1

Make sure you call the function like this:

SELECT * FROM get_map_data( ... );

And not like this:

SELECT get_map_data( ... );

Or you get the whole composite type as a single column.

Possible explanation

UUIDs can be written with enclosing curly braces ({}) in text representation. This happens to match the syntax for literal array constants, which use the same braces. This literal can be cast to uuid and uuid[] alike:

'{a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11}'

Which might introduce ambiguity. Just speculating, but I could imagine that some clients get confused with the syntax rules and concatenate element and array to a single datum.

Even this SQL Fiddle runs into a JDBC error! (While the same works with integer data.)

Better alternative

Either way, there is a lot of pointless effort in your function. Looping, unnesting and re-building arrays, ...
The whole shebang can be replaced with this simple and much faster query:

SELECT m.lid, tiles
FROM   map_layers  m
JOIN   layer_tiles l USING (lid)
WHERE  m.mid = $mid_val;

Wrap it into an SQL function if needed:

CREATE OR REPLACE FUNCTION get_map_data(_mid_val uuid)
  RETURNS SETOF map_data AS
$func$
SELECT m.lid, l.tiles
FROM   map_layers  m
JOIN   layer_tiles l USING (lid)
WHERE  m.mid = $1
$func$ LANGUAGE sql;