postgresql iterate n records - sql

Question

I have table student_list it contains student_id only one column and 100 rows.
i need to fetch 10-10 records concurrently and then perform some operation.
CREATE OR REPLACE FUNCTION loop_fetch()
RETURNS void AS
$BODY$
DECLARE
myrow student_list%rowtype;
cur1 CURSOR FOR SELECT * FROM student_list ;
BEGIN
OPEN cur1;
LOOP
-- i need to fetch rows based on limit
FETCH NEXT 10 FROM cur1 INTO myrow;
exit when myrow IS NULL;
INSERT INTO new_tbl SELECT myrow.student_id ;
END LOOP;
CLOSE cur1;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;
Any suggestion to achive this method - FETCH NEXT 5 FROM cur1 INTO myrow

Answer 1

Your code should not to work. FETCH NEXT 10 takes 10 rows from cursor, but INTO clause takes only first and other are lost. MyRow is composite variable - it can hold only one row.
I got a error:
ERROR: FETCH statement cannot return multiple rows
LINE 6: fetch 10 from r into re;
^
It is correct result.
The short and probably most correct solution is using FOR IN SELECT. This statement uses cursors internally, and it fetch 10 rows in first iteration, and 50 in other iterations.
So:
DECLARE r record;
BEGIN
FOR r IN SELECT * FROM student_list
LOOP
INSERT INTO newtbl VALUES(r.*);
END LOOP;
END;
does almost all what you want.
If you want to use cursors explicitly (and really you want to fetch in block), you need to use more cycles. The following code is little bit strange, and I write it here only for education - don't think so it has any benefit for practical life.
CREATE OR REPLACE FUNCTION loop_fetch()
RETURNS void AS
$BODY$
DECLARE
myrow student_list%rowtype;
cur1 CURSOR FOR SELECT * FROM student_list ;
rows int DEFAULT 0;
BEGIN
LOOP
-- i need to fetch rows based on limit
FOR myrow IN cur1
LOOP
INSERT INTO new_tbl SELECT myrow.student_id ;
END LOOP;
EXIT WHEN NOT FOUND;
rows := rows + 1;
EXIT WHEN rows = 10;
END LOOP;
END;
$BODY$
LANGUAGE plpgsql VOLATILE;
This code should to work, but it is crazy.
or you can use use FOR IN EXECUTE
do $$
declare
c cursor for select * from generate_series(1,100);
r record;
begin
-- *** UGLY CODE, DON'T DO IT!!! ***
open c;
-- iteration over FETCH is possible only via
-- dynamic SQL. FOR statement uses FETCH internally
-- by default, and is nonsense use FETCH 2x
for r in execute 'fetch 10 from c'
loop
raise notice '%', r;
end loop;
close c;
-- *** UGLY CODE, DON'T DO IT!!! ***
end;
$$;
Internally FOR IN query use a cursor. So FOR IN EXECUTE FETCH is reading via cursor from another cursor, what is performance nonsense and it really ugly code.
Important things - PostgreSQL has not tabular variables - so you cannot to assign more rows to one variable, and you cannot to fill more rows from one variable.
But your request looks like premature optimization. The most fast and most effective is command:
INSERT INTO newtbl SELECT studentId FROM student_list
SQL can process very well massive operations. 1M rows is nothing. Use cursors only when you really really need it.