Summary description:
id: select identifier
select_type: indicates the type of query.
table: the table of the output result set
partitions: the matching partition
type: the connection type of the table
possible_keys: the index that may be used in the query
key: the index actually used
key_len: the length of the index field
ref: the comparison between the column and the index
rows : The number of rows scanned (estimated number of rows)
filtered: the percentage of rows filtered by table conditions
Extra: description and explanation of the execution