MyISAM creates defect indexes on varchar, if rowcount is above some threshold between 480,000,000 and 490,000,000

DROP TABLE IF EXISTS test;

CREATE TABLE test (

rs	VARCHAR(20) PRIMARY KEY

) ENGINE=MyISAM DEFAULT CHARSET=latin1;

rs is in the form "rs12364789", max. length of the string is 12; always "rs" before a number

# myisamchk --keys-used=0 -rq test.MYI

... FLUSH

# mysqlimport --local --fields_escaped_by=\\ -u root -p test_db test.tsv

... FLUSH

# myisamchk --analyze --update-state --sort_buffer_size=56G  --recover --quick --quick --tmpdir=/mnt/WORK/tmp test.MYI

... FLUSH

RESULTS:

• with 480,000,000 rows everything is OK
• with 490,000,000 rows nothing will be found.
  EXPLAIN says: "Impossible WHERE noticed after reading const tables"

The row can still be found with IGNORE INDEX (PRIMARY)

If I build another table with only a non unique index on `rs` , EXPLAIN says it's using the index but the query won't find anything..

A server version from 3 or 4 Months ago worked OK.

Hope it helps

Thanks