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Does FORALL improve delete performance?

Oracle Database Tips by Donald BurlesonOctober 28,  2015

Question by Brannan:  I note that the FORALL operator is available for bulking of delete statements.  How does the context switch work with bulking of deletes?  I understand why forall is faster for inserts and updates, but with deletes there are no rows to transfer during the context switch.  Is there any performance difference between these two deletes, one using forall?

FORALL

forall i in 1 .. l_c1.count
delete from .....;

FOR..LOOP

for i in 1 .. l_c1.count loop
delete from .....;
end loop;

 

Answer: I'm a DBA, not a coder, but lets see what we can find on forall.

If you don't already have it, get Dr. Hall's book "PL/SQL Tuning Secrets", a great book for learning about PL/SQL bulking.

I would guess that the forall for delete reduces context switches, but it would not be as much as an insert.  The docs suggest that forall is done faster with deletes:

The FORALL statement in Example 11-2 sends all three DELETE statements to the SQL engine at once.

Example 11-2 Issuing DELETE Statements in a Loop

CREATE TABLE employees_temp AS SELECT * FROM employees;
DECLARE
TYPE NumList IS VARRAY(20) OF NUMBER;
depts NumList := NumList(10, 30, 70); -- department numbers
BEGIN
FORALL i IN depts.FIRST..depts.LAST
DELETE FROM employees_temp WHERE department_id = depts(i);
COMMIT;
END;
/

See my related note on PL/SQL bulk operator performance:


 

 

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