Oracle SQL - How to do massive updates more efficient and faster? - sql

Question

I'm trying to update 500.000 rows at once. I have a table with products like this:
+------------+----------------+--------------+-------+
| PRODUCT_ID | SUB_PRODUCT_ID | DESCRIPTION | CLASS |
+------------+----------------+--------------+-------+
| A001 | ACC1 | coffeemaker | A |
| A002 | ACC1 | toaster | A |
| A003 | ACC2 | coffee table | A |
| A004 | ACC5 | couch | A |
+------------+----------------+--------------+-------+
I've sets of individually statements, for example:
update products set class = 'A' where product_id = 'A001';
update products set class = 'B' where product_id = 'A005';
update products set class = 'Z' where product_id = 'A150';
I'm making a query putting one update statement below the other update statement and putting a commit statement each 1.000 rows.
It works fine (slow, but fine) but I wanna do it better if it can be possible in any way.
There is a better way to do this more efficient and faster?

Answer 1

One approach would be to create a temporary table holding your update information:
new_product_class:
product_id class
========== =====
A A001
B A005
Z A150
product_id should be an indexed primary key on this new table. Then you can do an UPDATE or a MERGE on the old table joined to this temporary table:
UPDATE (SELECT p.product_id, p.class, n.product_id, n.class
FROM product p
JOIN new_product_class n ON (p.product_id = n.product_id)
SET p.class = n.class
or
MERGE INTO product p
USING new_product_class n
ON (p.product_id = n.product_id)
WHEN MATCHED THEN
UPDATE SET p.class = n.class
Merge should be fast. Other things that you could look into depending on your environment: create a new table based on the old table with nologging followed by some renaming (should backup before and after), bulk updates.

Answer 2

Unless you have an index, each of your update statements scans the entire table. Even if you do have an index, there is a cost associated with the compilation and execution of each statement.
If you have a lot of conditions, and those conditions can vary, then I think Glenn's solution is clearly the way to go. This does everything in a single transaction, and there is no reason to run batches of 1,000 rows -- just do everything all at once.
If the number of conditions is relatively finite (as your example), and they don't change very often, then you can also do this as a simple case:
update products
set class =
case product_id
when 'A001' then 'A'
when 'A005' then 'B'
when 'A150' then 'C'
end
where
product_id in ('A001', 'A005', 'A150')
If it's possible your class field is already set to the correct value, then there is also great value in adding a condition to make sure you are not updating something to the same value. For example if this:
update products set class = 'A' where product_id = 'A001';
Updates 5,000 records, 4,000 of which are already set to 'A', then this would be significantly more efficient:
update products
set class = 'A'
where
product_id = 'A001' and
(class is null or class != 'A')