The anatomy of an SQL query
Every query to the database goes through several important stages of processing. Let's break down this process using the example of a query:
Step 1: Transport Subsystem
When you execute a query, the query string first enters the database's transport subsystem. This subsystem is responsible for managing the connection with the client and performs the initial authentication and authorization checks. If everything is in order, the query is passed to the next stage.
Step 2: Query Handler
The query enters the query handler, which consists of two main components:
- Query Parser: It breaks the SQL query into its components (SELECT, FROM, WHERE, etc.), checks for syntax errors, and creates a parse tree.
- Query Optimizer: After the query is parsed, the optimizer checks for semantic errors (e.g., whether the "users" table exists). It also determines the most efficient way to execute the query. As a result of the optimizer’s work, an execution plan is created, which describes how the query will be executed.
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