MyISAMIndex Statistics Collection
Storage engines collect statistics about tables for use by the optimizer. Table statistics are based on value groups, where a value group is a set of rows with the same key prefix value. For optimizer purposes, an important statistic is the average value group size.
MySQL uses the average value group size in the following ways:
To estimate how many rows must be read for each
To estimate how many rows a partial join will produce; that is, the number of rows that an operation of this form ...