MyISAM’s Index statistics collection is based on a set of rows with the same key prefix value. Here, these set of rows are called as value group. The statistics about the tables is used by the optimizer. The average value of the group size plays an improtant role. The group size is used estimate how many rows must be read for each ref access and how many rows will a particular join will produce. Myisam_stats_method system variable if set as global, affects statistics collection for all tables.