When will the index fail

1、 If there is... In the condition or, Even if there is a conditional index, it will not be used ( That's why we try to use as little or Why )

Be careful ： Want to use or, And want the index to work , Can only be or Each column in the condition is indexed

2、 For multi-column indexes , Not the first part of use , Index will not be used

3、like The query is based on % start

4、 If the column type is string , Be sure to quote the data in the condition , Otherwise, the index is not used

5、 Column to column contrast
In some table , Two columns a,b Are individually indexed , The following query conditions will not be indexed

select * from test where a = b

6、 There are calculations on the index column

select * from test where upper(name)='SUNYANG';

This is not going to be indexed , Because the query criteria are indexed
When the query condition does not have an index , You can still use the index

select * from test where name=upper('sunyang');
--INDEX RANGE SCAN

7、 Predicate operations

select * from table where id/2 = type_id

8、 Index can be used for range lookup , But behind the scope Condition will invalidate the index