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The built-in read-write-split mechanism is very basic. Every query which starts with SELECT is considered a read request to be sent to a MySQL slave server. All other queries (such as SHOW statements) are considered as write requests that are sent to the MySQL master server. The build-in behavior can be overruled using SQL hints, or a user-defined callback function.

The read-write splitter is not aware of multi-statements. Multi-statements are considered as one statement. The decision of where to run the statement will be based on the beginning of the statement string. For example, if using mysqli_multi_query to execute the multi-statement SELECT id FROM test ; INSERT INTO test(id) VALUES (1), the statement will be redirected to a slave server because it begins with SELECT. The INSERT statement, which is also part of the multi-statement, will not be redirected to a master server.