When trying to understand queries in the slow log, an interesting metric to look at is rows examined. Also, when trying to understand CPU spikes on a MySQL instance, the InnoDB Metric dml_reads and the global status Innodb_rows_read are interesting to check. In a similar way, when trying to gather information about which queries are loading the system, SUM_ROWS_EXAMINED from the| J-F Gagné's MySQL Blog
Sometimes, the MySQL Optimizer chooses a wrong plan, and a query that should execute in less than 0.1 second ends-up running for 12 minutes ! This is not a new problem: bugs about this can be traced back to 2014, and a blog post on the subject dates of 2015. But even if this is old news, because this problem recently came to my attention, it is a problem worth writing on. This| J-F Gagné's MySQL Blog