MySQL index optimization

1. Role of the index:

(1) Improve query speed

(2) Improve the sorting speed

(3) Increase the speed of group statistics

2. Parsing of the index:

Explain select * from T4 where a1=3 and A2>4 and a3= 5

3. Clustered index and non-clustered index

Non-clustered index used by the MyISAM storage engine

InnoDB uses clustered indexes (primary key usage), and other fields are indexed by sub-index

InnoDB The primary index file, the row data is called a clustered index, and the secondary index points to a reference to the primary key.

MyISAM, both primary and secondary indexes point to physical rows (disk location)

4. Index also Btree index and hash index

5. Summary

MySQL index optimization