Question

I have a table containing 60 million rows. The structure is like entryid, date, sourceid, detail, views. (entryid, date, sourceid, detail) is the PK, and I also have indexes for each field except views.

The problem is the cardinalities of the four indexes are zero, but I am sure they should not.

I wonder why is that? And does it mean the index doesn’t work?

Answer 1

It’s possible that the table statistics have not been updated.

See this page on optimizing MyISAM tables:

To help MySQL better optimize queries, use ANALYZE TABLE or run
myisamchk –analyze on a table after it has been loaded with data.
This updates a value for each index part that indicates the average
number of rows that have the same value. (For unique indexes, this is
always 1.) MySQL uses this to decide which index to choose when you
join two tables based on a nonconstant expression. You can check the
result from the table analysis by using SHOW INDEX FROM tbl_name and
examining the Cardinality value. myisamchk –description –verbose
shows index distribution information.

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The best way to determine whether an index is helping is to explain a query:

mysql> explain select 1;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)