[learning] database: MySQL query conditions have functions that lead to index failure. Establish functional indexes

Preface

• We all know mysql When where When there is a function query in the condition , Can cause indexes to fail
• for example ：

select * from test where DATE(swap_time) = DATE( date_sub( CURRENT_DATE, INTERVAL 1 DAY ) );

• use Explain Commands can be viewed sql Implementation plan of , Whether the index is used , Can pass possible_keys and rows It's easy to see
• In this case mysql 5.7 There are different solution suggestions before and after the version
• First, query the current mysql The version used

select VERSION();

mysql 5.7 Previous version optimization suggestions

select * from test where swap_time >= CURRENT_DATE and swap_time < date_add(CURRENT_DATE,INTERVAL 1 day);

mysql 5.7 Optimization suggestions for future versions

• stay 5.7 Later versions of the database support the establishment of functional indexes
• Create virtual columns

alter table test add column swap_time_index datetime GENERATED ALWAYS AS (DATE(swap_time));

• Re execution sql

select * from test where DATE(swap_time) = DATE( date_sub( CURRENT_DATE, INTERVAL 1 DAY ) );

• I found that the query speed has become faster
• utilize Explain View execution plan

Explain select * from test where DATE(swap_time) = DATE( date_sub( CURRENT_DATE, INTERVAL 1 DAY ) );

• It is found that the functional index is used

Reference address ：

mysql Build functional index
MySQL Explain Detailed explanation
MySQL Functional index and optimization
mysql How to view sql Statement execution time and efficiency