(1) I copy myTable with a name of myTable2.
(2) TRUNCATE TABLE mytable
(3) ALTER TABLE myTable ADD INDEX ( id)
(4) copy all records in myTable2 into myTable while myTable2 is looping with SELECT * FROM myTable order by id
will change the cardinality back to 1467 (if that is the number of records in the table).
To quote from the mySQL manual regarding what cardinality is:
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. The higher the cardinality, the greater the chance that MySQL uses the index when doing joins.