EllisLab/CodeIgniter — pull request #1338 by aphofstede (2012-05-07 21:36:04+00:00)
Title: Autocommit already handled by MySQL implicitly.

See MySQL documentation (http://dev.mysql.com/doc/refman/5.5/en/commit.html):
"To disable autocommit mode implicitly for a single series of statements, use the START TRANSACTION statement:

With START TRANSACTION, autocommit remains disabled until you end the transaction with COMMIT or ROLLBACK. The autocommit mode then reverts to its previous state."

So removing them saves 2 database calls per transaction..