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I have x row count, but all index cardinarlities are much greater than x, how is this possible? If I understand correctly, index cardinality is the number of unique rows of that index.

+1  A: 

From SHOW INDEX:

Cardinality
An estimate of the number of unique values in the index. This is updated by running ANALYZE TABLE or myisamchk -a. Cardinality is counted based on statistics stored as integers, so the value is not necessarily exact even for small tables.

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