I've got a dedicated mysql server (5.5.44) which runs now about 202 days. Between Friday at about 11:00 a.m. and Saturday at about 3:30 a.m. I've got a very high load on that server (my monitoring shows a system load from up to 25). I've checked serveral log files but can't find anything what goes wrong. I've also checked the mysql-slow.log and there I discovered, that there are queries logged, which are normally never logged. The first query from that file shows me
Query_time: 11.387071 Lock_time: 0.000045 Rows_sent: 8 Rows_examined: 9975109
When I take the exact same query now and run it directly, I got in the explanation that only 19023 rows have to be examined. The slow query log is full of such examples.
So it looks to me, that some internal indexes from mysql have failed. Could that be? Or is there anything internal maintenance running in mysql?
By now, the mysql server is fast as before. But what happened in the time between friday and saturday?