Understanding pdo mysql transactions

Question

The PHP Documentation says:

If you\'ve never encountered transactions before, they offer 4 major features: Atomicity, Consistency, Isolation and Durabili

Answer 1

Judging by the given conditions (a solitary DML statement), you don't need a transaction here, but a table lock. It's a very common confusion.

You need a transaction if you need to make sure that ALL your DML statements were performed correctly or weren't performed at all.

Means

• you don't need a transaction for any number of SELECT queries
• you don't need a transaction if only one DML statement is performed

Although, as it was noted in the excellent answer from Shadow, you may use a transaction here with appropriate isolation level, it would be rather confusing. What you need here is table locking. InnoDB engine lets you lock particular rows instead of locking the entire table and thus should be preferred.

In case you want the salary to be 1200 - then use table locks.

Or - a simpler way - just run an atomic update query:

UPDATE employees SET salary = salary + 1000 WHERE name = ?

In this case all salaries will be recorded.

If your goal is different, better express it explicitly.

But again: you have to understand that transactions in general has nothing to do with separate scripts execution. Regarding your topic of race condition you are interested not in transactions but in table/row locking. This is a very common confusion, and you better learn it straight:

• a transaction is to ensure that a set of DML queries within one script were executed successfully.
• table/row locking is to ensure that other script executions won't interfere.

The only topic where transactions and locking interfere is a deadlock, but again - it's only in case when a transaction is using locking.