Mysql database index (InnoDB engine)

About b+ Please refer to the related concepts of tree by yourself , This article mainly distinguishes non unique indexes 、 unique index 、 Joint index ( Composite index )、 Differences between overlay indexes .

unique index On the leading node are the primary key and pointer , Leaf nodes store complete data .

Non unique index As a result b+ Trees , But leaf nodes do not store specific data , It stores the primary key value , You need to query back to the table in the primary key index .

Composite index Is to index multiple fields , Accordingly, it will also create b+ Trees , Leaf nodes store specific data . Composite index (A,B,C) Query criteria (A,B,C),(A),(A,B) Can use composite index , According to other examples (A,C) Some use indexes , It involves table query backfill , You can use explain To analyze .

Overlay index Not an index created , It's just the name of a phenomenon , If there is no back table query for other types of index queries , Call it Overlay index , Can pass explain Query view Extra Field is Using index, For example, the direct hit record of a single index is Overlay index . If the query in the composite index is basically equal to the query or other queries that can directly hit the record , It does not involve back table query , This is also the case Overlay index .