mysql execution process

MySQL is a popular relational database management system. Its execution process can be divided into three main parts: parsing phase, optimization phase and execution phase. During the execution of MySQL, the input SQL statement will first be parsed into an abstract syntax tree by the parser, then optimized to determine the optimal query plan, and finally the query operation will be executed and the results will be returned. This article will introduce the specific processes and key technologies of these three stages respectively.

1. Parsing phase

MySQL's parser is responsible for converting the input SQL statement into an abstract syntax tree (AST), and converting the AST into an internal representation (such as a "query block" or " expression tree"). During the parsing process, MySQL will also perform the following operations:

1. Grammar check: MySQL will check whether the entered SQL statement conforms to the language specification.
2. Lexical analysis: MySQL will break down the input statement into words (for example, operators and identifiers) and identify the types of these words.
3. Semantic analysis: MySQL analyzes the entered statements to determine whether they are valid. For example, semantic analysis will confirm whether the table name exists, whether the column name is correct, etc.

The output of the parser is a syntax tree that captures the syntax and semantic information of the input SQL statement.

2. Optimization phase

The optimizer of the SQL statement is responsible for deciding how to query the data most effectively. The optimizer first analyzes the query syntax tree to generate potential query plans, and then uses a series of cost estimation techniques to evaluate the performance cost of each query plan and select an optimal solution.

The optimizer's task is to make queries faster and more efficient. To do this, it performs the following operations:

1. Logical optimization: The optimizer optimizes the expression tree and reorganizes predicates, columns, and operators to create more efficient query statements. New expression tree.
2. Physical optimization: MySQL will select a best query plan based on data storage, index information, MySQL configuration parameters and other conditions. MySQL uses decision-making strategies such as cost estimation, dynamic programming, greedy algorithms, and stochastic optimization.
3. Caching strategy: MySQL will maintain the query cache to improve query efficiency. When a query cache hits, MySQL returns the results immediately without loading the data from disk.

The output of the optimizer is the optimal query plan, which includes all operations required during the execution phase.

3. Execution Phase

The executor is responsible for executing the initial query plan, processing the results and returning the results. The query plan is generated by the optimizer and sent to the executor. The executor will perform the following operations:

1. Query plan scan: The executor executes the query plan by scanning tables and indexes. It converts each query block into execution operators such as physical operators, selection operations, projection operations, etc., and then performs the operations.
2. Cache-based access: The executor will obtain and cache the data that has been read from memory. If the query results are already in the cache, the executor fetches the results from the cache instead of re-reading the results from disk.
3. Transaction control: The executor will start, commit or roll back a transaction based on whether the SQL statement contains a transaction indicator. The executor maintains transaction isolation levels, locks, etc.

The output of the executor is the result of the execution, including query results, the number of changed records and other information.

Summary

The execution phase of MySQL consists of three main parts: parsing phase, optimization phase and execution phase. The parser first converts the input SQL statement into an abstract syntax tree, and then the optimizer selects the optimal query plan and sends it to the executor. The executor converts the execution plan into physical operators, performs query operations, and returns execution results. The execution process of MySQL is different from other database management systems because it uses some cost estimation and optimization techniques for query plan generation. This makes MySQL very good in large data processing and has been widely used.

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