The type of explain in MySQL

|  All | Full table Scan

|  Index | Index full scan

|  Range | Index range scanning, common language <,<=,>=,between and other operations

|  Ref | Use a non-unique index scan or a unique index prefix scan to return a single record that often appears in an associated query

|  Eq_ref | Similar to ref, the difference is that a unique index is used, and the associated query using the primary key

|  Const/system | A single record, the system processes other columns in the matching row as constants, such as a primary key or a unique index query

|  null | MySQL does not access any tables or indexes and returns results directly

# # # #由上至下, more efficient

The type of explain in MySQL