Q:

Why did my MySQL Index Cardinality get Zeroed out? 

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? 

A:

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   I'd just expect that they'd be out of date.