PostgreSQL function returning result set from nested queries

Question

I have to write a PL/pgSQL stored procedure which has two inputs.
The first input should go into a select which returns a set of timestamps.

With these returned timestamps I intend to loop over a second select, which takes the second input of the function and returns a single row I have to collect in the iteration of the loop. I then assemble the output of the second select and return that as a result set.

FUNCTION (Input A B)
SELECT FROM T1 WHERE X = A INTO RS1.
LOOP VAL OVER RS1
   Select FROM T2 WHERE Y = B AND Z = VAL INTO RS2
END LOOP
RETURN ASSEMBLED RS2

Can somebody point me to a working skeleton on which I can work on, btw. what are the relevant parts I need from the documentation? I think I need:

• RETURNS TABLE () AS to declare the result set

• A CURSOR to loop over the second query, using the results of the first query

Answer 1

Just try to use Common Table Expressions instead of trying to iterate explicitly over rows using pgpsql code :

with T1_vals as (
  select val from T1 where X = A
), T2_vals as (
  select T2.val from T2, T1_vals where T2.Z = T1_vals.val and T2.Y = B
) select val from T2_vals;

As a general rule : be careful when you're tempted to iterate explicitly over values, as SQL offers generally simplier solutions with much better performance.

Answer 2

You don't need a cursor, a nested function call or a loop.
You don't need a CTE , either.

Use a very basic query with a JOIN :

SELECT t2.*
FROM   t1
JOIN   t2 ON t2.z = t1.val 
WHERE  t1.x = _input_a
AND    t2.y = _input_b;

Set-based solutions are typically superior to a procedural approach in SQL. It's a rare occasion that looping is the best solution.