LP #1258135: pt-deadlock-logger introduces a noise to MySQL

**Reported in Launchpad by Roman Vynar last update 10-07-2014 12:12:31

Hello,

pt-deadlock-logger introduces a noise to MySQL because it does:
INSERT IGNORE INTO `percona`.`deadlocks` (`server`,`ts`,`thread`,`txn_id`,`txn_time`,`user`,`hostname`,`ip`,`db`,`tbl`,`idx`,`lock_type`,`lock_mode`,`wait_hold`,`victim`,`query`)
VALUES ...

every minute on every deadlock that appears under SHOW ENGINE INNODB STATUS.

For example, say a deadlock happened 1 year ago, and MySQL keeps it in the memory and pt-deadlock-logger will INSERT it into percona.deadlocks table every minute all the time until restart. This introduces a noise.
If we expect to see no writes on the master, pt-deadlock-logger should not be a problem and should not introduce those unnecessary writes.

The suggestion is to do SELECT to check whether a row exists in percona.deadlocks table and only do INSERT otherwise.

Thanks.