random dive
A technique for quickly estimating the number of different values in a column (the column's cardinality). InnoDB samples pages at random from the index and uses that data to estimate the number of different values. This operation occurs when each table is first opened.
Originally, the number of sampled pages was fixed at 8; now, it is determined by the setting of the innodb-stats-sample-pages parameter.
The way the random pages are picked depends on the setting of the innodb_use_legacy_cardinality_algorithm parameter. The default setting (OFF) has better randomness than in older releases.
See also cardinality.