ALL	Full table scan
index	Full index scan
range	Range search, commonly found in <, <=, >, >=, between
index_subquery	There may be multiple equality in the connection item
unque_subquery	Appears in the connection, the connection items correspond one by one
index_merge	It means that the query uses more than two indexes
ref_or_null	Similar to ref, it is just that there is another null judgment in the result set of the first query.
fulltext	Full-text index, full-text index has high priority, and when it exists at the same time as ordinary index, full-text index will be used first.
ref	Secondary index matching, full value matching
eq_ref	Primary key index or unique index term matching, full value matching
const	At most, only one record matches
system	There is only one row of data in the table or an empty table, which is a special case of the const type. And it can only be used for myisam and memory tables.
NULL	Data that can be obtained without tables