Problem:-
In a single-table update, when there is a constant equality predicate
on a key.The bug reporter is expecting explain output type as const.

Analysis:-
In query like
create table tb(id int primary key , c int); 

mysql> explain update tb set c=2 where id=1\G
id    1
select_type    SIMPLE
table    tb
type    range            
possible_keys    PRIMARY
key    PRIMARY
key_len    4
ref    NULL
rows    1                
Extra    Using where 

EXPLAIN does not print type: eq_ref/const even though we have an equality 
predicate on a Unique key field.

However, we can't  show const/eq_ref as range access is used, and 
single-table UPDATE/DELETE "optimizer" does not even consider index 
lookups. The proper fix for this bug would be to make single-table U/D go 
through the normal query optimizer, but that's a complex task(for which WL#6367 
Single code path for multi-table and single-table UPDATE(DELETE) is created).

But to improve readability, we are changing value of the ref column to ref:const.