I see plan of query:

EXPLAIN SELECT *
FROM `subscribers`
WHERE state =4
AND date_added < '2010-12-23 11:47:45'

It shows:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  subscribers     range   state_date_added    state_date_added    9   NULL    8   Using where

Here is indexes of table:

Table   Non_unique  Key_name    Seq_in_index    Column_name     Collation   Cardinality     Sub_part    Packed  Null    Index_type  Comment
subscribers     0   PRIMARY     1   subscriber_id   A   382039  NULL    NULL        BTREE    
subscribers     0   email_list_id   1   email_address   A   191019  NULL    NULL        BTREE    
subscribers     0   email_list_id   2   list_id     A   382039  NULL    NULL        BTREE    
subscribers     1   FK_list_id  1   list_id     A   10  NULL    NULL        BTREE    
subscribers     1   state_date_added    1   state   A   12  NULL    NULL        BTREE    
subscribers     1   state_date_added    2   date_added  A   8128    NULL    NULL        BTREE    

The last two lines describes index what is supposed for the query. Why is there in extra column using where? Even If I fetch only state and date_added column, it has in extra column: Using where; Using index. I understand why it has using index, but I don't understand Using where here.