How SQL Server Handles SQL Batch Execution

When SQL Server compiles and executes a SQL batch, it does not compile each line of code individually. Instead, it processes the batch as a whole. This creates an execution plan for the entire query or set of statements in the batch.

Here’s how it works in more detail:

1. Compilation vs. Execution

• Compilation: SQL Server parses the entire batch, validates the syntax and semantics, and generates a query execution plan.
• Execution: The relational engine executes the plan step-by-step. It interacts with the storage engine to fetch or modify data.

2. Processing a SQL Batch

When a SQL batch (a group of T-SQL statements) is submitted, SQL Server performs the following steps:

a. Parsing:

• The entire batch is parsed as a single unit to ensure syntax correctness.
• If syntax errors are detected, the batch fails at this step.

b. Binding:

• SQL Server resolves object references (e.g., tables, columns) for all statements in the batch.
• Checks are performed to ensure all objects exist and that the user has appropriate permissions.

c. Optimization:

• The optimizer evaluates the batch as a whole to generate execution plans for the statements.
• For queries, it considers indexes, joins, and statistics to produce the most efficient plan.

d. Execution:

• Statements are executed sequentially, following the compiled plan.

3. Behavior of Each Line in a Batch

While SQL Server processes the batch as a whole, execution of individual statements happens sequentially. For example:

DECLARE @Counter INT = 1;
SELECT * FROM Employees;
UPDATE Employees SET Salary = Salary * 1.10 WHERE Department = 'HR';

• SQL Server compiles this batch into a single execution plan.
• Execution occurs one statement at a time, but only after the batch is fully compiled.

4. Considerations for Errors

• Compilation Errors: If any syntax or binding error occurs during compilation, the entire batch fails and no statement is executed.
• Execution Errors: If an error occurs during execution (e.g., division by zero), subsequent statements in the batch may still execute unless explicitly handled (e.g., with a TRY...CATCH block).

5. When Each Statement is Compiled Separately

There are exceptions where SQL Server might recompile a specific part of a batch:

• Dynamic SQL: When executing a query using EXEC or sp_executesql, the query is compiled separately at runtime.
• Stored Procedures: Each execution of a stored procedure involves separate compilation (or reuse of a cached plan).
• Recompilation Events: Changes to the database schema, statistics, or query hints may trigger recompilation of individual statements.

Conclusion

SQL Server does not compile each line of code individually within a batch. Instead, it parses, binds, and optimizes the entire batch into an execution plan, then executes the statements sequentially. This approach improves performance by minimizing overhead and enabling holistic optimization for the batch.