week 9

9.1 Overview of Query Processing

When you submit an SQL query, the DBMS does not simply 'read' and execute it directly. It goes through a sophisticated multi-stage process to determine the most efficient way to retrieve the requested data. This is query processing.

THE QUERY PROCESSING PIPELINE:

Stage	Description
1. Parsing and Translation	SQL query is parsed for syntax errors and translated into an internal algebraic representation (relational algebra expression tree).
2. Query Optimisation	The query optimiser generates multiple equivalent execution plans and selects the one with the lowest estimated cost (I/O, CPU, memory).
3. Code Generation	The chosen plan is compiled into executable code.
4. Query Execution	The execution engine runs the plan, accessing data from disk/memory as needed.
5. Result Return	The final result set is returned to the user or application.