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I have an older web application that uses a MySQL Database (MYISAM). I noticed recently that the performance of the application was drastically reduced. After checking on my indexes, I noticed that the cardinality for all of them was reporting zero.
I was able to fix this by doing an ANALYZE TABLE on each table.
But I’m curious, what causes the cardinality to get zeroed out in the first place?
From memory, some repair table operations wipe the statistics and you need to ANALYZE TABLE to get them back.
Other than that, I can’t think of any reason why they should be zeroed. I’d just expect that they’d be out of date.