Create a table like this:

CREATE TABLE itg_passwd (
  Username char(20) NOT NULL default '',
  Password char(20) NOT NULL default '',
  PRIMARY KEY  (Username),
  KEY zzzz (Password(4))
) TYPE=MyISAM;

And insert some data;
insert into itg_passwd values('1','h1'), ('2','h2'), ('3','h1/a');

Then Query index of itg_passwd:
show index from itg_passwd;

I can see Cardinality of index  zzzz is 'NULL' ! (It should be 3)

Why??? 

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