possible_keys:
displays the index in this table may use one or more.
Queries related to the field if there is an index, the index will be listed, but not necessarily the actual query
key:
the index of actual use. If NULL, then do not use the index
If the query uses the coverage index, the index and query selet field overlap occurs only in the key list.
Cover index: the number of fields and the order of the fields indexed exactly match queries
The key_len:
number of bytes used in the index, said length of the index may be used in the query is calculated by the column. Without loss of accuracy, a length as short as possible
key_len maximum possible length values of the index fields displayed, not the actual length , i.e. calculated on the basis key_len table definition is not retrieved by the inner
