Don't ask me again why MySQL hasn't left the index? For these reasons, I'll tell you all

In the work , I often have problems like this , I am clearly in MySQL The table is indexed , Why execute SQL The index is not used when querying ？

Same article SQL Sometimes queries use indexes , Sometimes the index is not used , What's the matter ？

The reason may be that the index is invalid , There are several reasons for failure , See if you have stepped on a similar pit ？

1. Data preparation ：

There is such a user table , stay name Build an index on the field ：

CREATE TABLE `user` (
  `id` int NOT NULL AUTO_INCREMENT COMMENT ' Primary key ',
  `name` varchar(255) DEFAULT NULL COMMENT ' full name ',
  `age` int DEFAULT NULL COMMENT ' Age ',
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB COMMENT=' User table ';

2. Explain Detailed explanation ：

Want to view a SQL Whether index is used ？ What type of index is used ？

have access to explain keyword , see SQL Implementation plan . for example ：

explain select * from user where id=1;

You can see type=const, Indicates that the primary key index is used .

explain All of the type The types are as follows ：

3. Cause of failure

1. Data type implicit conversion

name The fields are varchar type , If we use data type query , Data type conversion will occur , Although not wrong , But the index cannot be used .

explain select * from user where name=' A lamp ';

explain select * from user where name=18;

2. Fuzzy query like With % start

explain select * from user where name like ' Zhang %';

explain select * from user where name like '% Zhang ';

3. or Index not used before and after

although name The field is indexed , however age Field has no index , Use or The whole table will be scanned when .

# or Index not used before and after , Causes a full table scan 
explain select * from user where name=' A lamp ' or age=18;

4. Joint index , The first column index is not used

If we were （name,age） On , Set up a joint index , But only age Field , Indexes are also unavailable .

Use union index , The leftmost matching principle must be followed , First use the first column of fields , Then use the second column field .

CREATE TABLE `user` (
  `id` int NOT NULL AUTO_INCREMENT COMMENT ' Primary key ',
  `name` varchar(255) DEFAULT NULL COMMENT ' full name ',
  `age` int DEFAULT NULL COMMENT ' Age ',
  PRIMARY KEY (`id`),
  KEY `idx_name_age` (`name`,`age`)
) ENGINE=InnoDB COMMENT=' User table ';

5. Calculate in the index field

If we calculate in the index column , Indexes are also unavailable .

#  Calculate on the primary key index , Causes a full table scan 
explain select * from user where id+1=2;

6. Use functions on index fields

If we use functions in index columns , Indexes are also unavailable .

7. The index optimizer is wrong

Same article SQL Sometimes queries use indexes , Sometimes the index is not used , What's the matter ？

This may be the result of optimizer selection , You will choose whether to use the index according to the amount of data in the table .

When most of the table name It's all a light , Use at this time name=' A lamp ' Make a query , Will indexes be used ？

The index optimizer will think , Using an index is not as fast as a full table scan , Simply do not index .

Of course, we think the optimizer is not optimized properly , You can also use force index Force index .

Summary of knowledge points ：