PostgreSQL dynamic fetch foward count value - sql

Question

I have the following function used to retrieve a batch of Ids from a table. This function is being used as the OFFSET and LIMIT clauses seem to offer poor performance.
CREATE OR REPLACE FUNCTION getBatch(_workloadId VARCHAR, _offSet INT, _limit INT)
RETURNS SETOF NUMERIC AS $$
DECLARE
c SCROLL CURSOR FOR
SELECT id FROM workload WHERE workload_id = $1 ORDER BY id ASC;
BEGIN
OPEN c;
MOVE FORWARD $2 IN c;
RETURN QUERY FETCH FORWARD 10 FROM c;
END;
$$ LANGUAGE plpgsql;
I want the FETCH FORWARDcount to be passed in as a parameter, but I'm unable to find a way to do this. Referencing $3 does not work, I've also tried the following:
EXECUTE 'RETURN QUERY FETCH FORWARD ' || $3 || ' FROM c;';
Any help would be greatly appreciated.

Answer 1

You are confusing SQL cursors with PL/pgSQL cursors, which are similar but not the same. In particular, there is no FETCH FORWARD count in PL/pgSQL:
The direction clause can be any of the variants allowed in the
SQL FETCH command except the ones that can fetch more than one
row; namely, it can be NEXT, PRIOR, FIRST, LAST, ABSOLUTE
count, RELATIVE count, FORWARD, or BACKWARD.
In PL/pgSQL you can only fetch one row at a time and process (or return) it.
There is also a disclaimer in the manual:
Note: This page describes usage of cursors at the SQL command level. If you are trying to use cursors inside a PL/pgSQL function,
the rules are different.
You could open that cursor in PL/pgSQL and loop to return rows. But that's only relevant if you want to fetch multiple distinct pieces from a big cursor to save overhead. Else a plain FOR loop (with automatic cursor) or a simple SELECT with OFFSET and LIMIT are certainly faster. Cursors are primarily meant to be returned to and used by the client:
CREATE OR REPLACE FUNCTION getbatch_ref(_cursor refcursor, _workload_id text)
RETURNS refcursor
LANGUAGE plpgsql AS
$func$
BEGIN
OPEN $1 SCROLL FOR
SELECT id
FROM workload
WHERE workload_id = $2
ORDER BY id;
RETURN $1;
END
$func$;
You can use this function in SQL:
BEGIN;
SELECT getbatch_ref('c', 'foo');
MOVE FORWARD 10 IN c;
FETCH FORWARD 10 FROM c;
ROLLBACK; -- or COMMIT;
You could also just use plain SQL:
BEGIN;
DECLARE c SCROLL CURSOR FOR
SELECT id
FROM workload
WHERE workload_id = 'foo'
ORDER BY id;
-- OPEN c; -- only relevant in plpgsql
-- The PostgreSQL server does not implement an OPEN statement for cursors;
-- a cursor is considered to be open when it is declared.
MOVE FORWARD 10 IN c;
FETCH FORWARD 10 FROM c;
ROLLBACK; -- or COMMIT;