Let's discuss MariaDB/MySQL isolation levels. Understanding them means to know how your DBMS can protect you from anomalies, and how to apply potentially important optimisations.

1. Transaction Isolation Levels
are your friends. Don’t be scared.

2. € whoami
● Federico Razzoli
● Freelance consultant
● Writing SQL since MySQL 2.23
hello@federico-razzoli.com
● I love open source, sharing,
Collaboration, win-win, etc
● I love MariaDB, MySQL, Postgres, etc
○ Even Db2, somehow

3. Transactions
START TRANSACTION;
-- check if the product is available
SELECT qty
FROM product
WHERE id = 333
LOCK IN SHARE MODE;
-- create the order
INSERT INTO order
(user_id, status, product_id)
VALUES
(42, 'NEW', 333);
-- decrease quantity in stack
UPDATE product
SET qty = qty - 1
WHERE id = 333
COMMIT;

4. Transactions
START TRANSACTION;
SELECT qty
FROM product
WHERE id = 333
FOR UPDATE;
INSERT INTO order
(user_id, status, product_id)
VALUES
(42, 'NEW', 333);
UPDATE product
SET qty = qty - 1
WHERE id = 333
COMMIT;
START TRANSACTION;
SELECT qty
FROM product
WHERE id = 333
FOR UPDATE;
INSERT INTO order
(user_id, status, product_id)
VALUES
(42, 'NEW', 333);
UPDATE product
SET qty = qty - 1
WHERE id = 333
COMMIT;

5. Transaction Isolation Levels
● READ UNCOMMITTED
● READ COMMITTED
● REPEATABLE READ ← default
● ( SERIALIZABLE )

6. READ UNCOMMITTED
● We see data that is not COMMITted
○ And maybe will never be
For large queries/transactions, this is a major optimisation
When should I use it?
● Statistics on many rows
● Removing old rows that no one is supposed to be reading
● Read / modify session data
● Inserting data

7. READ COMMITTED
● Only see COMMITted data
● But every query acquires a new view on data
● It’s like running separate transactions, but the queries succeed or fail
altogether
Why should I use it?
● “Gap locks”
○ Big INSERT … SELECT
● Single statement transactions (autocommit=1)
● Can be a great optimisation with big INSERT ... SELECTs

8. REPEATABLE READ
● Only see COMMITted data
● A view on data is acquired at the beginning of the transaction
● Cannot see changes made by others in the middle of the transaction
Why should I use it?
● Purchase a product
● Reserve a ticket
● etc

9. SERIALIZABLE
● Like REPEATABLE READ, but all SELECTs are locking
● Same as using SELECT ... LOCK IN SHARE MODE

10. Thank you kindly!
https://federico-razzoli.com
info@federico-razzoli.com