What is meaning of "func" in "ref" column of EXPLAIN plan

[Leo Hopkins]

Hello,

I have an EXPLAIN plan and is as follows :


*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: abc
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 4773821
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: xyz
type: ref
possible_keys: Id,date_idx
key: Id
key_len: 195
ref: func
rows: 2
Extra: Using where
2 rows in set (0.10 sec)

mysql>


Now in the EXPLAIN plan in the "ref" there is the value as "func". I want to understand what exactly "func" in this case means and some details regarding this value.

Thanks in advance.