What is the Using temporary status in EXPLAIN and how to avoid it?

Using temporary indicates that the need to create temporary tables in MySQL queries is common in ORDER BY using DISTINCT, GROUP BY or non-indexed columns. You can avoid the occurrence of indexes and rewrite queries and improve query performance. Specifically, when Using temporary appears in EXPLAIN output, it means that MySQL needs to create temporary tables to handle queries. This usually occurs when: 1) deduplication or grouping when using DISTINCT or GROUP BY; 2) sort when ORDER BY contains non-index columns; 3) use complex subquery or join operations. Optimization methods include: 1) creating appropriate indexes for columns in ORDER BY and GROUP BY; 2) rewrite the query, such as changing complex subqueries to join operations; 3) using overlay indexes to obtain data directly from the index. Through these strategies, the use of temporary tables can be significantly reduced and query efficiency can be improved.

introduction

When we dig into MySQL query optimization, the EXPLAIN command is a powerful tool in our hands, which can help us explore the execution plan of SQL query. In this process, Using temporary state often confuses us, even fears, because it means that MySQL needs to use temporary tables when executing queries, which often leads to performance issues. Today, we will unveil the mystery of Using temporary , explore the reasons behind it, and share some practical experiences and tips to help you avoid it.

By reading this article, you will learn about the definition and role of Using temporary , analyze in-depth how it works, learn how to identify and solve this problem with actual code examples, and master some performance optimization and best practices.

Review of basic knowledge

Before we get started, let's quickly review the basics related to EXPLAIN and Using temporary . EXPLAIN command is a tool provided by MySQL to analyze the execution plan of SQL statements. It will return detailed information to help us understand the execution process of query.

Using temporary is a flag in the EXPLAIN output, indicating that when executing a query, MySQL needs to create a temporary table to store intermediate results. This temporary table may exist in memory or be written to disk, depending on the size of the data and the configuration of the system.

Core concept or function analysis

Definition and function of Using temporary

When Using temporary appears in EXPLAIN output, it means that MySQL needs to create a temporary table during query execution. This usually occurs in the following situations:

• When using DISTINCT or GROUP BY clause, MySQL needs to deduplicate or group the results.
• ORDER BY clause contains non-indexed columns, and MySQL needs to sort the results.
• Some complex subquery or join operations are used.

While Using temporary itself does not necessarily mean poor query performance, it does increase the complexity and resource consumption of the query. Therefore, understanding why it appears and trying to avoid it is an important step in optimizing queries.

How it works

When MySQL executes a query that requires Using temporary , it follows:

1. Create a temporary table : According to the query requirements, MySQL creates a temporary table in memory or on disk.
2. Populate data : Populate query results into a temporary table.
3. Operation temporary table : Sorting, deduplication or other operations on the temporary table.
4. Return result : Finally, the result in the temporary table is returned to the user.

Although this process seems simple, it actually involves operations at multiple levels such as MySQL's storage engine, memory management, and disk I/O. Therefore, the creation and operation of temporary tables may become performance bottlenecks.

Example of usage

Basic usage

Let's look at a simple example to illustrate the emergence of Using temporary :

 EXPLAIN SELECT DISTINCT name FROM users ORDER BY age;

In this query, MySQL needs to deduplicate name and sort it by age . Both operations require temporary tables, so Using temporary temporary will appear in EXPLAIN output.

Advanced Usage

Sometimes, Using temporary comes about because we use complex subqueries or join operations. Let's take a look at a more complex example:

 EXPLAIN SELECT * FROM orders o
JOIN (
    SELECT customer_id, MAX(order_date) as last_order_date
    FROM orders
    GROUP BY customer_id
) last_orders ON o.customer_id = last_orders.customer_id AND o.order_date = last_orders.last_order_date;

In this query, the subquery needs to group the orders table, which will lead to the occurrence of Using temporary .

Common Errors and Debugging Tips

In actual development, the emergence of Using temporary is often because we do not make full use of indexes, or the query design is not reasonable enough. Here are some common errors and debugging tips:

• Not using the right index : Make sure that the columns involved in your query have the right index, especially those in ORDER BY and GROUP BY .
• Complex subqueries : Try to avoid using complex subqueries, you can try to rewrite them as a join operation.
• Overuse of DISTINCT : If you do not need to deduplicate, try to avoid using DISTINCT .

Performance optimization and best practices

In practical applications, the key to avoiding Using temporary is to optimize query design and make full use of indexes. Here are some practical experiences and best practices:

• Optimized indexing : Create appropriate indexes for columns in ORDER BY and GROUP BY , which can significantly reduce the use of temporary tables.
• Rewrite query : Sometimes, by rewriting the query, the creation of temporary tables can be avoided. For example, rewrite complex subqueries into join operations.
• Use overlay indexes : If possible, try to use overlay indexes, so that you can get data directly from the index without creating temporary tables.

Let's look at an optimized example:

 -- Original query EXPLAIN SELECT DISTINCT name FROM users ORDER BY age;

-- Optimized query CREATE INDEX idx_age_name ON users(age, name);
EXPLAIN SELECT name FROM users USE INDEX (idx_age_name) ORDER BY age;

In this example, we successfully avoid the occurrence of Using temporary by creating a joint index idx_age_name and using this index in the query.

In actual projects, I once encountered a complex report query, which caused extremely poor query performance due to a large number of GROUP BY and ORDER BY operations involved. By analyzing EXPLAIN output, it was found that Using temporary is the main bottleneck. Finally, we successfully reduce the query time from minutes to seconds by rewriting the query and optimizing the index.

In general, although Using temporary is a common phenomenon, through reasonable query design and index optimization, we can completely minimize its impact. I hope this article can provide you with some valuable insights and practical experience to help you go further on the road of MySQL query optimization.

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