Sabtu, 23 Juni 2012

A little "side-effect" of having autocommit off in MySQL

. Sabtu, 23 Juni 2012 .

Back-story: A developer came to me and wanted explanation for a weird behavior in MySQL. They inserted a record (to InnoDB table), committed, and after receiving a message (on another application) tried to read that inserted record immediately, but the newly inserted record was not found. Problem only happened in production, but not always (quite frequently).
After comparing the MySQL parameter files between production and development environments I discovered that in production autocommit was disabled to make MySQL behave more like Oracle. This setting was removed from development after we rebuilt the environment (to use multiple MySQL instances with Oracle Clusterware, instead of one large MySQL instance), but the rebuild was not yet done in production.
The default transaction level for MySQL InnoDB is REPEATABLE READ (unlike Oracle, that has READ COMMITTED as default), that means that the SELECT query always returns the data at the time point when the transaction was started. If autocommit is off, then the first issued select statement will open the transaction and any subsequent select statement will return the data at the time point when the first select was issued, until transaction is ended with COMMIT/ROLLBACK. If autocommit is enabled, SELECT statement is run in a self-contained transaction, ending with COMMIT, so the end result is like READ COMMITTED isolation level in Oracle.

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