Interview shock 60: what will cause MySQL index invalidation?

In order to verify  MySQL  Under what circumstances will the index fail , We can use  explain  Execute the plan to analyze the specific scenario of index failure .

explain  Use as follows , Just look up the  SQL  Add... Before  explain  Key words can be used , As shown in the figure below ：

In the columns of the above query results , Our main observation  key  This column ,key  This column represents the index actually used , If  NULL  The index is not used , Otherwise, the index is used .

All the results listed above are explained as follows ：

• id —  Select the identifier ,id  The higher the priority, the higher , The first to be executed ;

• select_type —  Represents the type of query ;

• table —  Output result set table ;

• partitions —  Matching partition ;

• type —  Indicates the connection type of the table ;

• possible_keys —  When representing a query , Possible indexes ;

• key —  Indicates the index actually used ;

• key_len —  Length of index field ;

• ref—  Comparison of columns and indexes ;

• rows —  Approximate estimated number of lines ;

• filtered —  Percentage of rows filtered by table criteria ;

• Extra —  Description and description of implementation .

One of the most important is  type  Field ,type  The value types are as follows ：

• all —  Scan the full table data ;

• index —  Traverse index ;

• range —  Index range lookup ;

• index_subquery —  Use... In subqueries  ref;

• unique_subquery —  Use... In subqueries  eq_ref;

• ref_or_null —  Yes  null  To optimize the index  ref;

• fulltext —  Use full text indexing ;

• ref —  Find data using a non unique index ;

• eq_ref —  stay  join  Use primary key or unique index association in query ;

• const —  Put a primary key in  where  The following is used as a conditional query , MySQL  The optimizer can turn this query optimization into a constant , How and when to transform , It depends on the optimizer , This ratio  eq_ref  Be more efficient .

• Create test tables and data

• In order to demonstrate and test that situation will lead to index invalidation , Let's create a test table and corresponding data ：

 --  Create table
 drop table if exists student;
 create table student(
 id int primary key auto_increment comment ' Primary key ',
 sn varchar(32) comment ' Student number ',
 name varchar(250) comment ' full name ',
 age int comment ' Age ',
 sex bit comment ' Gender ',
 address varchar(250) comment ' Home address ',
 key idx_address (address),
 key idx_sn_name_age (sn,name,age)
 )ENGINE=InnoDB DEFAULT CHARSET=utf8;
 --  Add test data
 insert into student(id,sn,name,age,sex,address) 
 values(1,'cn001',' Zhang San ',18,1,' Gao Laozhuang '),
 (2,'cn002',' Li Si ',20,0,' Huaguoshan '),
 (3,'cn003',' Wang Wu ',50,1,' Waterfall cave ');


There are... In total in the current table  3  An index , As shown in the figure below ：

PS： The following content of this article is based on  MySQL 5.7 InnoDB  Data engine .

Index failure 1： Non leftmost match

The leftmost matching principle refers to , Queries that start with the leftmost field can use a union index , Otherwise, you will not be able to use the union index .

The field order of our union index is  sn + name + age, We assume that their order is  A + B + C, The following union indexes are used as follows ：

As can be seen from the above results , If the matching fields start from the leftmost, the upper union index can be used , such as ：

• A+B+C

• A+B

• A+C

• among ：A  Equals field  sn,B  Equals field  name,C  Equals field  age.

and  B+C  But you can't use the union index , This is the left most matching principle .

Index failure 2： Error fuzzy query

Fuzzy query  like  The common usage of  3  Kind of ：

• Fuzzy match followed by any character ：like ' Zhang %'

• Fuzzy match any preceding character ：like '% Zhang '

• Fuzzy matching before and after any character ：like '% Zhang %'

And this  3  There are only two kinds of fuzzy queries  1  Three query methods can be used to index , The specific implementation results are as follows ：

Index failure 3： Column operation

If the index column uses an operation , Then the index will fail , As shown in the figure below ：

Index failure 4： Using functions

If any... Is used in the query column  MySQL  The provided function will invalidate the index , For example, the following columns use  ifnull  The execution plan after the function is as follows ：

Index failure 5： Type conversion

If the index column has a type conversion , Then I won't go to the index , such as  address  Is a string type , When querying, you set  int  A value of type will invalidate the index , As shown in the figure below ：

Index failure 6： Use  is not null

When used in a query  is not null  It can also cause indexes to fail , and  is null  The index will be triggered normally , As shown in the figure below ：

summary

Lead to  MySQL  Common scenarios for index invalidation are as follows  6  Kind of ：

• The union index does not satisfy the leftmost matching principle .

• The first fuzzy query is an indefinite matching character .

• The index column is involved in the operation .

• The index column uses the function .

• Index column has type conversion .

• Index columns use  is not null  Inquire about .

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number .

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