MySQL index optimization miscellaneous

Index push down

Index push down （Index Condition Pushdown,ICP）： For auxiliary federated indexes , Normally, the leftmost prefix is used ,SELECT * FROM employees WHERE name like ‘LiLei%’ AND age = 22 AND position =‘manager’（ With name,age,position Index for Federation ） This situation will only go name Field index , Because according to name After filtering the fields , In the resulting index line age and position Is chaotic , Can't make good use of indexes .
stay MySQL5.6 Previous version , This query can only match the name in the union **‘LiLei’ start ** The index of , Then take the primary keys corresponding to these indexes back to the table one by one , Find the corresponding record in the primary key index , In comparison age and position Whether the values of these two fields match .
stay MySQL5.6 Index push down optimization is introduced , During index traversal , Judge all the fields contained in the index first , Filter out the unqualified records before returning to the table , It can effectively reduce the times of returning to the table . After using index push down , According to the above query, the name matched in the union index is ‘LiLei’ After the first index , It also filters in the index age and position These two fields , Take the primary key corresponding to the filtered index id Go back to the table and check the data of Zheng hang .

Index push down will reduce the number of table returns , about innoDB The table index push down of the engine can only be used for secondary indexes ,innoDB Primary key index of （ Cluster index ） The tree leaf node stores the entire row of data , So at this time, the index push down will not have the effect of querying the whole row of data less .

Why range lookup MySQL No index push down optimization is used
It should be MySQL The result set of the range lookup filter is considered too large ,like kk% In most cases, look at , The filtered result set is relatively small , So here MySQL Choose to give like KK% Index push down optimization , Of course , It's not absolute , Have you got anything to do like KK% It doesn't necessarily lead down . Related to optimizer processing

Force Index

EXPLAIN SELECT * FROM employees force index(idx_name_age_position) WHERE name > 'LiLei' AND age = 22 A ND position ='manager';

add force index, But I haven't seen much use of it .

in and or When the amount of table data is large, the index will be used , When there are few table records, full table scanning will be selected

like KK% In general, I will go to the index