The ref Column

This column shows which columns or constants from preceding tables are being used to look up values in the index named in the key column. Here’s an example that shows a combination of join conditions and aliases. Notice that the ref column reflects how the film table is aliased as f in the query text:

mysql> EXPLAIN
    -> SELECT STRAIGHT_JOIN f.film_id
    -> FROM sakila.film AS f
    ->    INNER JOIN sakila.film_actor AS fa
    ->       ON f.film_id=fa.film_id AND fa.actor_id = 1
    ->    INNER JOIN sakila.actor AS a USING(actor_id);
...+-------+...+--------------------+---------+------------------------+...
...| table |...| key                | key_len | ref                    |...
...+-------+...+--------------------+---------+------------------------+...
...| a     |...| PRIMARY            | 2       | const                  |...
...| f     |...| idx_fk_language_id | 1       | NULL                   |...
...| fa    |...| PRIMARY            | 4       | const,sakila.f.film_id |...
...+-------+...+--------------------+---------+------------------------+...