Cardinality of INDEX ??

[halojoy]

 Indexes:
Keyname Type     Cardinality Field
uname   UNIQUE    0           uname
id      INDEX     None        id

In my table,
UNIQUE 'uname' has cardinality=0
INDEX 'id' has cardinality=None

Questions:
1. What is Cardinality ?
2. Is Cardinality important, something to bother with ?
3. How can I change / set Cardinality ?

.

[tsinka]

Hi,

the cardinality can't be adjusted.
The cardinality is the number of unique values in this index. In your case the table seems to be empty. I somewhere read that this value isn't updated automatically but by executing "myisamchk -a" or the query "ANALYZE TABLE". This isn't true with our MySQL installation(s). The value increases automatically each time a record gets inserted (at least the cardinality of auto_increment columns).

Thomas