Query Processor

The Query Processor is a fundamental part of the Database Engine responsible for handling SQL queries from start to finish. Its primary role involves receiving a query request, parsing it to understand the syntax and structure, optimizing the execution plan for efficiency (considering factors like indexes and data distribution), and finally executing the plan to retrieve or manipulate data from the database storage. It acts as the intelligent intermediary between user requests and the raw data stored on disk.

The Query Processor operates through a multi-stage pipeline: 1) **Parsing & Validation**: The query string is broken down (parsed) into a structured format (often an abstract syntax tree) and checked for syntactic and semantic errors. 2) **Optimization**: The processor analyzes the parsed query and database statistics (e.g., table sizes, index availability) to generate multiple potential execution plans. It estimates the cost (in terms of I/O, CPU, memory) for each plan and selects the most efficient one. 3) **Execution**: The chosen plan is sent to the execution engine, which interacts with the storage manager to read/write data, perform joins, sorts, and aggregations, and finally return the result set to the user or application.