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What is the index in MySQL?

Index is also called "key" in mysql. It is a kind of mark used by mysql storage engine to quickly find records. Index plays a very important role in improving query speed. When the amount of data in the table is large enough, the role of index becomes more and more important.

Creating and maintaining indexes takes a lot of hard disk space and time, but it can greatly improve query speed.

Before using the index, you need to pay attention.

As much as possible to use some simple data types, ×× data type than string, processing overhead will be much less, because string comparison is more complex. In MySQL, time should be stored with built-in date and time data types, not strings; and IP addresses should be stored with integer data types.

Avoid nulls as much as possible Specify not null for every field (unless you want to store NULL) Columns with nulls are difficult to query optimize in MySQL because they complicate indexes, index statistics, and comparison operations. You should replace null with zero, a special value, or an empty string.

Don't overuse indexes.

The frequent condition behind where after query statement is more suitable for index.

It is not recommended to add indexes to fields with too concentrated values. Even if you add indexes, it will not improve the efficiency much (such as gender: male/female).

6. Select * from users where YEAR(adddate)

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