How to create an index in mysql

MySQL is a widely used relational database that plays an important role in data processing. It supports many operations, including index creation. Index is one of the important mechanisms in the database to improve query efficiency. When using MySQL, indexing is important. This article will introduce the concept of MySQL index and how to create it.

1. The concept of index
   The index is a data structure used for fast search in the database. By storing keywords and their location information in the data table in the index, you can quickly find relevant data rows and improve query efficiency. In MySQL, indexes can be divided into two types, B-tree indexes and hash indexes.
2. B-tree index
   B-tree is a multi-layered tree structure. It is a commonly used index data structure. In a B-tree, each node contains a key that points to the next node. In MySQL, B-tree indexes can be applied to all data types. By default, each table in MySQL automatically generates a B-tree index named PRIMARY to ensure the uniqueness of each row.

When creating an index, you should check the data type and length of the index column to ensure that it meets the requirements. In order to ensure the efficiency of the index, it is recommended to use the EXPLAIN command that comes with MySQL to view the statement execution plan. If the entire table is scanned, it is recommended to optimize the statement or add an index.

3. Hash index
   Hash index is a data structure indexed by the hash value of the keyword. In MySQL, hash indexes only work on in-memory tables.

Hash indexes can speed up lookups, but in some specific cases it may make queries slower. For example, hash indexes cannot be applied where sorting is required. In MySQL, the usage of hash indexes is very low, except when processing in-memory tables.

4. Index Creation
   In MySQL, use the CREATE INDEX statement to create an index. It is assumed here that a Person table needs to be created, including the id, name and age fields.

To create a B-tree index, you can use the following SQL statement:

CREATE INDEX idx_name ON Person(name(50));

The above statement creates an index named idx_name in the name field of the Person table, where name (50) means that the maximum length of this field is 50.

It should be noted that indexing will occupy disk and memory resources, so the number of indexes should be determined based on the actual situation and access mode requirements. It is recommended to test the performance first before deciding whether to use it.

5. Index optimization
   In a database, how to use indexes is a very critical optimization strategy. Several ways to optimize the index are as follows:

5.1 Index coverage scan
Index coverage scan means that during the query process, only the index itself is used without table access. This can greatly improve query efficiency.

For example, if you need to find all the names of people older than 30 in the Person table, you can use the following SQL statement:

SELECT name FROM Person WHERE age > 30;

If the age field has been indexed, you can use the index coverage scan To optimize the query:

SELECT name FROM Person WHERE age > 30 AND name IS NOT NULL;

Since the index contains the name field, the index can be used directly when querying without further access to the table.

5.2 The leftmost prefix principle
The leftmost prefix principle means that when using a multi-column index, the index can be used for optimization only when the first column of the index meets the query conditions. This means that if you need to perform fuzzy queries on multiple columns, you should consider indexing by merging multiple columns into a single column.

5.3 Index selectivity
The selectivity of the index refers to the proportion of the index used in the query. If selectivity is low, many rows will be scanned, which will reduce query efficiency. The appropriate index should be selected based on the number of queries and the needs of the access pattern.

6. Summary
   Index is an important mechanism for optimizing queries in the MySQL database. When using indexes, you must choose appropriate indexes based on different needs, and optimize indexes through coverage scans, leftmost prefix principles, and index selectivity to improve query efficiency. The ultimate goal is to improve the query performance of the database and make query operations more efficient and faster.

The above is the detailed content of How to create an index in mysql. For more information, please follow other related articles on the PHP Chinese website!