Possible causes of index failure

Index is mainly to improve the efficiency of data query , But in some cases, indexing will reduce query efficiency , Increase cost .

1） The data magnitude is too small . The index itself also takes up a certain amount of space , Too little data , Using an index doesn't make much sense .
2） Frequently updated fields . The construction of index 、 maintain 、 Deleting also consumes system resources .
3） Fields with low discrimination . Like gender , Generally, there are only two choices , It's difficult for the index to query the desired data only by gender .

Get down to business , The possible causes of index invalidation are as follows ：
Suppose the primary key column pri
Secondary index column mul mul1 mul2
No index column noidx no_index_col

1） The query criteria include or And the field column does not contain an index

select * from test where mul=1 or noidx=2;

Secondary index +or+ Indexed columns ： I will take the index first , But columns without indexes will be scanned in the whole table , So it's better not to go through the index , Directly scan the whole table .
If or There are indexes before and after , Then maybe go to the index , Or maybe it's not indexed .

2） Implicit conversion leads to index invalidation
Suppose that when a mobile phone number column is created num varchar(15)
In the query ,where The condition column is followed by a string , Use quotation marks

select * from test where num='13911111111';

If not quoted , Is the number , Character types do not match , Don't walk index .

3）like And % Is the prefix's non overlay index

% in front , Don't walk index

select * from test where mul like "%abc";

% Not in front , Go to the index

select * from test where mul like "abc%";

Use overlay index ,% in front , Also go to the index

select pri,mul from test where mul like "%abc";

4） The union index is created and the query does not meet the leftmost matching principle , Index may fail , It may still work . There are many examples of index failure , Here is an example , Express The index is still valid .

Notice the union index I created ,name before ,id After .

CREATE TABLE a(
id INT NOT NULL,
NAME VARCHAR(5)
)ENGINE=INNODB,CHARSET=utf8

ALTER TABLE a ADD INDEX idx_name_id(NAME,id);

Obtaining an execution plan ：

DESC SELECT * FROM a WHERE NAME='a' AND id <2;

Obtaining an execution plan ：

DESC SELECT * FROM a WHERE id <2 AND NAME='a'; 

Obtaining an execution plan ：

DESC SELECT * FROM a WHERE id =3;

5） Perform numerical operations on indexed columns , Index failure

CREATE TABLE `user`( 
`id` INT(11) NOT NULL AUTO_INCREMENT, 
`userId` VARCHAR(32) NOT NULL,  
`age` INT(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_age`(`age`) USING BTREE
)ENGINE=INNODB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

Get index execution plan ：

DESC SELECT * FROM USER WHERE age+1=8;

6） Used in index It's not equal to The symbol of , Index may fail
Include <> != not in
It should be noted that , If it is a clustered index column , Use the above symbols , Still can walk the index .

7） use or The two connected contain null Index field , Don't walk index .

for example ：

select * from test where mul1 is null or mul2 is not null;

however , A single index contains null Field , It's indexed .

select * from test where mul1 is null;

8） No query criteria , Or the column of the query criteria has no index , No index
for example ：

select * from test;
select * from test where no_index_col=3;

9）Mysql When the optimizer analysis of believes that the full table scan is faster than the index , For example, the result set of the query is not less than the total data 25%-30% when , It believes that there is no need to index .

10） The index itself fails .
Although the index has the ability to maintain itself , But when the content of the data table is modified and updated frequently , It is also possible that the index fails , You need to delete the index , Re index .

Reference article ：

Oldguo- Benchmarking class -MySQL-lesson04- Index and execution plan

Ten miscellaneous problems of index failure