Wednesday, April 13, 2016

Query Rewrite Plugin and Binlog for Replication

Starting with MySQL 5.7 we introduced the Query Rewrite Plugin. That tool is really useful for changing queries. Of course the best location to modify the query is the source code of the application, but this is not always possible. Either the application is not under your control or queries are generated from a framework like Hibernate and sometimes it is hard to change the query generation.
If you are interested in details about the Query Rewrite Plugin, I recommend this blogpost from the MySQL Engineering:
Recently I was asked how this works in replication environments. Which query goes into the binlog?

If you are using the Rewriter plugin that comes with MySQL 5.7, the answer is easy: This plugin only supports rewriting SELECT queries. SELECT queries don't get into the binlog at all. Simple.

But you might write your own preparse or postparse plugin. In that case you can define the behavior with the server option --log-raw. See documentation here:
You can either bring the original query to the binlog or the rewritten query. So all flexibility you need. However be aware that --log-raw also affects logging of passwords in the general log file. With --log-raw passwords are written in plain text to the log files. So consider this side effect when switching --log-raw on or off.

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