Why isn’t MySQL using any of these possible keys?

Another issue you may be encountering is a data type mis-match. For example, if your column is a string data type (CHAR, for ex), and your query is not quoting a number, then MySQL won’t use the index.

SELECT * FROM tbl WHERE col = 12345; # No index
SELECT * FROM tbl WHERE col="12345"; # Index

Source: Just fought this same issue today, and learned the hard way on MySQL 5.1. 🙂

Edit: Additional information to verify this:

mysql> desc das_table \G
*************************** 1. row ***************************
  Field: das_column
   Type: varchar(32)
   Null: NO
    Key: PRI
Default: 
  Extra: 
*************************** 2. row ***************************
[SNIP!]

mysql> explain select * from das_table where das_column = 189017 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: das_column
         type: ALL
possible_keys: PRIMARY
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 874282
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from das_table where das_column = '189017' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: das_column
         type: const
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 34
          ref: const
         rows: 1
        Extra: 
1 row in set (0.00 sec)