Query Processor Architecture

At the core of the SQL Server database engine are two major components:

• The storage engine: The storage engine is responsible for reading data between the disk and memory in a manner that optimizes concurrency while maintaining data integrity.
• The relational engine (also called the query processor): The query processor, as the name suggests, accepts all queries submitted to SQL Server, devises a plan for their optimal execution, and then executes the plan and delivers the required results.

Queries are submitted to SQL Server using SQL or Structured Query Language (or T-SQL, the Microsoft SQL Server extension to SQL). Because SQL is a high-level declarative language, it only defines what data to get from the database, not the steps required to retrieve that data or any of the algorithms for processing the request. Thus, for each query it receives, the first job of the query processor is to devise, as quickly as possible, a plan that...