Question

Will out-of-transaction SELECT see only committed data?

UPD. My database is MySQL

UPD. I’m interested in simple SELECT statement which comes without explicit transaction. What data will it see? How can I change it?

Answer 1

I assume you are asking regarding InnoDB, since MyISAM does not support transactions.

There is no such thing as out of transaction, even with the default autocommit=1, every statement for itself is a transaction.

The answer to your question depends on what you mean by only committed data.

Say we have a table with a row:

INSERT table (id, Counter, LastUpdates) VALUES (8, 100, '2011-03-08 22:04:00');

While session A is modifying the row:

START TRANSACTION;
UPDATE table SET Count = 101, LastUpdates = NOW() WHERE id = 8;
SLEEP(100);
COMMIT;

If session B issues a select before session A commits the transaction, what will happen is dependent on the transaction isolation level of session B.

• READ UNCOMMITTED: session B will read the newly modified row (counter = 101).
• READ COMMITTED / REPEATABLE READ (default): Session B will see the latest committed row (counter = 100).
• SERIALIZABLE: session B will block until session A commits.

Here is how a one can change the session transaction level without declaring a transaction.

SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;