MySQL InnoDB SELECT...LIMIT 1 FOR UPDATE Vs UPDATE ... LIMIT 1

Question

I have a table v_ext in a MySQL with InnoDB engine:
- id: primary key
- code: pre-generated list of codes (say 1000 codes are generated randomly)
- user_id: initially NULL

When a user purchase an item, they receive a code. I need to update the table to populate the user_id column. I have two options:

START TRANSACTION;
SELECT id FROM v_ext WHERE user_id IS NULL LIMIT 1 FOR UPDATE; -- return id 54 for ex.
UPDATE v_ext SET user_id=xxx WHERE id=54;
COMMIT;

or

UPDATE v_ext SET user_id=xxx WHERE user_id IS NULL LIMIT 1;

Is the second option safe if I have thousands of users purchasing at the same time? If so, is it correct to assume this second option is better for performance since it needs only one query?

Answer 1

Since I didn't get an answer, I started doing benchmarking. My criteria are as follows:

• 20,000 pre-generated codes
• Use of Apache ab command with 20,000 requests, 100 concurrency: ab -n 20000 -c 100
• Servlet -> EJB (JPA 2.0 EclipseLink, JTA) to perform the update in DB (as it will be through a JSF action in real situation)
• 2 versions of the Servlet, one with option 1 (SELECT ... FOR UPDATE ), and one with option 2 (UPDATE ... LIMIT 1)
• Stopped Glassfish, hit the tested Servlet manually 5 times to warm it up, reset all to NULL to user_id
• Tests are run 3 times each and average is provided

Results:

SELECT ... FOR UPDATE; UPDATE ... :

Concurrency Level:      100
Time taken for tests:   758.116 seconds
Complete requests:      20000
Failed requests:        0
Write errors:           0
Row updated:            20000

UPDATE.... LIMIT 1:

Concurrency Level:      100
Time taken for tests:   773.659 seconds
Complete requests:      20000
Failed requests:        0
Write errors:           0
Row updated:            20000

So at least on my system, the option with 2 queries seems more efficient than the one query. I didn't expect that :)