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CHANGE MASTER TO changes the parameters that the slave server uses for connecting to the master server, for reading the master binary log, and reading the slave relay log. It also updates the contents of the master.info andrelay-log.info files. To use CHANGE MASTER TO, the slave replication threads must be stopped (use STOP SLAVE if necessary).
CHANGE MASTER TO deletes all relay log files and starts a new one, unless you specify RELAY_LOG_FILE orRELAY_LOG_POS. In that case, relay log files are kept; the relay_log_purge global variable is set silently to 0.