Last night I was asked about index cardinality. One of the members of the North Texas MySQL Users Group was using phpMyadmin and noticed an element marked 'cardinality' and asked me what it meant. And I will admit I was stumped.

The manual says:

ANALYZE TABLE determines index cardinality (as displayed in the Cardinality column of SHOW INDEX output) by doing ten random dives to each of the index trees and updating index cardinality estimates accordingly. Because these are only estimates, repeated runs of ANALYZE TABLE may produce different numbers. This makes ANALYZE TABLE fast on InnoDB tables but not 100% accurate because it does not take all rows into account.

MySQL uses …