In Oracle the default transaction isolation is READ_COMMITTED. In MySQL the default is REPEATABLE_READ. Because MySQL also has READ_COMMITTED I have seen in more then one production MySQL environment a transaction isolation of READ_COMMITTED. The explanation and ultimately incorrect assumption is the default in Oracle is READ_COMMITTED so we made that the default in MySQL.
I’m not going to discuss the specific differences of these isolation levels (see reference lines below) except to say it that READ_COMMITTED in Oracle more closely relates to the MySQL default of REPEATABLE_READ and not READ_COMMITTED. Just because the same term for a common feature exists, don’t assume the underlying functionality is the same or that either or both actually conform to the SQL ANSI standard.
While switching your MySQL environment to READ_COMMITTED is possible, there is still conjucture if this actually provides any performance improvement. There are different cases of improving locking contention, in one case Heikki Tuuri the creator of InnoDB suggests READ_COMMITTED may overcome an adjacent range gap locking contention problem while in a tpcc-like benchmark a far greater number of deadlocks were detected.
I will close by stating two facts. When changing the MySQL transaction isolation from the default of REPEATABLE_READ you are using a code path that is less tested and not used as frequently to the millions of default MySQL installations, and you are also required to change the default replication format, again a code path less tested and potential a significant increase in I/O load.
- Ask Tom – On Transaction Isolation Levels
- Understanding Innodb Transaction Isolation
- READ-COMMITED vs REPETABLE-READ in tpcc-like load
- The InnoDB Transaction Model and Locking
- Replication Formats.
About “Don’t Assume”
“Don’t Assume” is a series of posts to help the Oracle DBA understand, use and appreciate the subtle differences and unique characteristics of the MySQL RDBMS in comparison to Oracle. These points as essential to operate MySQL effectively in a production environment and avoid any loss of data or availability.
The MySQLCamp for the Oracle DBA is a series of educational talks all Oracle DBA resources should attend. Two presentations from this series IGNITION and LIFTOFF will be presented at the MySQL Users Conference 2010 in Santa Clara, April 2010 This series also includes JUMPSTART and VELOCITY. If you would like to here these presentations in your area, please contact me.