Why is this query using where instead of index?

EXPLAIN EXTENDED SELECT  `board` . * 
FROM  `board` 
WHERE  `board`.`category_id` =  '5'
AND  `board`.`board_id` =  '0'
AND  `board`.`display` =  '1'
ORDER BY  `board`.`order` ASC

The output of the above query is

id  select_type table   type    possible_keys   key key_len ref rows    filtered    Extra
1   SIMPLE  board   ref category_id_2   category_id_2   9   const,const,const   4   100.00  Using where

I'm a little confused by this because I have an index that contains the columns that I'm using in the same order they're used in th开发者_开发知识库e query...:

category_id_2   BTREE   No  No 
category_id 33  A       
    board_id    33  A   
    display 33  A   
    order   66  A   

The output of EXPLAIN can sometimes be misleading.

For instance, filesort has nothing to do with files, using where does not mean you are using a WHERE clause, and using index can show up on the tables without a single index defined.

Using where just means there is some restricting clause on the table (WHERE or ON), and not all record will be returned. Note that LIMIT does not count as a restricting clause (though it can be).

Using index means that all information is returned from the index, without seeking the records in the table. This is only possible if all fields required by the query are covered by the index.

Since you are selecting *, this is impossible. Fields other than category_id, board_id, display and order are not covered by the index and should be looked up.

It is actually using index category_id_2.

It's using the index category_id_2 properly, as shown by the key field of the EXPLAIN.

Using where just means that you're selecting only some rows by using the WHERE statement, so you won't get the entire table back ;)