How to create function(table_name) that returns a set of the table

I have a set of tables that share the same fields (some timestamps and users names used for version control). I need to create a function with dynamic sql that uses a table_name and a timestamp as arguments and returns a set of the same table.

The query I need is a bit more complicated, but for my question the following example should be enough.

This is how I'm trying to create the function:

CREATE OR REPLACE FUNCTION "vsr_versioning_at_time"(_t regclass
                                                  , _d timestamp without time zone)
RETURNS SETOF _t AS
$
BEGIN
    EXECUTE 'SELECT DISTINCT ON (gid) * 
        FROM '|| _t || 
        'WHERE vrs_start_time <= '|| _d ||
        'ORDER BY gid, vrs_start_time DESC';
END
$
LANGUAGE plpgsql;

The creation of this function fails since _t is not recognized by the RETURN SETOF _t.

You cannot use a parameter name (_t) as return type.

Use polymorphic types instead:

CREATE OR REPLACE FUNCTION vsr_versioning_at_time(_t anyelement, _d timestamp)
  RETURNS SETOF anyelement AS
$func$
BEGIN
   RETURN QUERY EXECUTE format('
       SELECT DISTINCT ON (gid) * 
       FROM   %s
       WHERE  vrs_start_time <= $1
       ORDER  BY gid, vrs_start_time DESC'
     , pg_typeof(_t)
     )
   USING _d;
END
$func$  LANGUAGE plpgsql;

And use RETURN QUERY EXECUTE to actually return the results.

Call (important!):

SELECT * FROM vsr_versioning_at_time(NULL::mytable, '2014-04-18 12:00');

A lot more details in this related answer on SO (last chapter):

Refactor a PL/pgSQL function to return the output of various SELECT queries

StackExchange Database Administrators Q#63477, answer score: 3