BUG #8393: "ERROR: failed to locate grouping columns" on grouping by varchar returned from function

The following bug has been logged on the website:

Bug reference:      8393
Logged by:          Evan Martin
Email address:      postgresql@realityexists.net
PostgreSQL version: 9.2.4
Operating system:   Windows 7 x64 SP1
Description:

version(): PostgreSQL 9.2.4, compiled by Visual C++ build 1600, 64-bit


Run the following:


CREATE OR REPLACE FUNCTION test_group_by()
RETURNS TABLE (my_col varchar(5))
AS $BODY$
    SELECT 'hello'::varchar(5);
$BODY$ LANGUAGE sql STABLE;


SELECT my_col
FROM test_group_by()
GROUP BY 1;




Expected result:


'hello'


Actual result:


ERROR:  failed to locate grouping columns


Interestingly, if the function is marked "VOLATILE" it works. Casting the
result to "text" also makes it work.