What are the conditions of index invalidation?

Column to column contrast

In some table , There are two columns （mobile and phone） They all have separate indexes , The following queries do not go through the index ：

select * from test where mobile = phone;

In this case, it is considered that it is better to scan the whole table .

Index column exists NULL value

Mysql Allow in field null when , There is null value , Will it affect the index usage

NOT Conditions

When indexing , An entry will be created for each index , If the query range is equivalent or range query , The index can find the corresponding entry according to the query condition . But when the query condition is not , It is difficult to locate the index , This kind of query conditions are ：<>、not、in、not exists.

select * from test where id in (1,3,4,6);

LIKE wildcard

like How to avoid invalidation by using indexes

The conditions include functions

Try not to use functions on index columns , as follows ：

select * from test where upper(name) = ‘ZHANGSAN’;

This is not going to be indexed , Because the index may be different at build time and after calculation , Unable to locate index .
But if the query condition is not calculating the index column , Then the accident can be indexed , as follows ：

select * from test where name = upper(‘zhangsan’);

Such functions also have ：to_char、to_date、to_number、trunc etc. .

The leading column of composite index is distinguished greatly

MySQL Leading column property of Composite Index

Conversion of data types

When the query condition has an implicit transformation , The index will fail . For example, in the database id Deposit is number type , But in the query , But it uses the following form ：

select * from test where id = ‘1’;

Connect By、Level

oracle In the database connect by and level Usage of

Predicate operations

We said on it , Cannot perform a function operation on an index column , This also includes the predicate operation of adding and subtracting multipliers , It also invalidates the index . as follows ：

select * from test where id/2 = 1;

At this time, you will find out id by 2 The record of , Will invalidate the index . In this case, the index can be used ：

select * from test where id = 1*2;

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