In Oracle SQL, query compilation is the process by which Oracle prepares an SQL statement for execution by converting it into a low-level, efficient execution plan. The key stages of SQL compilation include:

1. Parsing: Oracle checks the syntax and semantics of the query, ensuring that database objects (tables, columns) are valid and that the query is permissible. If successful, Oracle creates a parse tree; otherwise, it raises an error.

2. Binding: Oracle binds the query’s references (tables, columns) to actual database objects and checks the data types of bind variables. In PL/SQL, bind variables are mapped to actual values.

3. Optimization: Oracle’s optimizer analyzes different execution paths and selects the most efficient plan based on factors like indexes, data statistics, and available system resources. This stage determines whether the query will use methods like index scans or full table scans.

4. Code Generation: The chosen execution plan is converted into machine code or bytecode that interacts with the database to fetch or manipulate data.

5. Execution: Oracle executes the compiled query and retrieves or modifies the data. The compiled plan is stored in the library cache for future reuse, improving performance.

Key Benefits of Compilation:

Performance: Optimized execution plans ensure the query runs efficiently with minimal resources.

Reusability: Cached execution plans are reused for similar queries, reducing compilation overhead and improving performance.

Example: For a query like SELECT first_name, last_name FROM employees WHERE employee_id = 101;, Oracle parses the query, binds the employee_id value, optimizes the query for performance (possibly using an index), generates machine code, and executes it to retrieve the result.