[saltcured]

Right, I'm aware of table-returning functions and the general idea of functions as table sources in a FROM clause. I think the biggest gap is not having complementary table-consuming function signatures and ways to use functions as table sinks other than aggregate function calls.

I'd also want to allow a function to be both a table source and a table sink, which I think means that you might need a sub-query like syntax to supply the source that will be consumed by the function. This is a vague idea and not a well-defined language proposal:

    CREATE OR REPLACE FUNCTION
    -- invent a table-sink func signature?
    func1( TABLE t1(a int, b text, c text) )
    RETURNS TABLE (x int, y boolean) AS $$
      -- access input set via declared name
      SELECT 
        a, b < c
      FROM t1
    $$ LANGUAGE SQL;

    CREATE OR REPLACE FUNCTION
    func2( TABLE t1(a int, b boolean) )
    RETURNS int AS $$
      SELECT a WHERE b
    $$ LANGUAGE SQL;

    -- implicit positional matching of func input cols?
    SELECT f1r.x, f1r.y
    FROM func1( SELECT e1, e2, e3 FROM ...) AS f1r;

    -- explicit input mapping to override positions?
    SELECT f1r.x, f1r.y
    FROM func1( SELECT e1, e2, e3 INTO b, a, c FROM ...) AS f1r;

    -- function composition
    SELECT func2(func1( SELECT e1, e2, e3 FROM ...));