When EXPLAIN shows that a table is accessed using the [eq_]ref access type, we're used to look at the ref column to see where MySQL gets the value to look up from. Usually, we see either "const" if the value is provided as a constant in the query or a column name if the value is read from a column in an already read table:

EXPLAIN 

SELECT acc_rec.cust_id, acc_rec.amount, acc_rec.invoicenumber

FROM accounts_receivable as acc_rec

WHERE acc_rec.cust_id=1;

+----+-------------+---------+------+---------+-------+

| id | select_type | table   | type | key     | ref   |

+----+-------------+---------+------+---------+-------+

| 1  | SIMPLE      | acc_rec | ref  | cust_id | const |

+----+-------------+---------+------+---------+-------+


EXPLAIN 

SELECT acc_rec.cust_id, acc_rec.amount, acc_rec.invoicenumber,

       customers.firstname, customers.lastname, customers.phone

FROM accounts_receivable AS acc_rec JOIN customers AS cust

ON acc_rec.cust_id = customers.cust_id;

+----+-------------+---------+--------+---------+-----------------+

| id | select_type | table   | type   | key     | ref             |

+----+-------------+---------+--------+---------+-----------------+

| 1  | SIMPLE      | acc_rec | ALL    | NULL    | NULL            |

| 1  | SIMPLE      | cust    | eq_ref | PRIMARY | acc_rec.cust_id |

+----+-------------+---------+--------+---------+-----------------+

But what if ref shows the value "func"? In this case, the value used as input to [eq_]ref is the output of some function. A few examples:

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