The main relational database product from Microsoft, SQL Server, is a complicated piece of software with a straightforward purpose: to process SQL queries and produce results as soon as feasible. Gaining an understanding of SQL Server's low-level workings can significantly boost the effectiveness and performance of your database applications. The goal of this blog is to clarify the procedure SQL Server goes through when handling your queries.

Parsing and Normalization
First, when a SQL query arrives, it undergoes parsing and normalization.

• Parsing: This phase involves scanning the incoming SQL text and dividing it into individual keywords, expressions, and identifiers. Invalid syntax would lead to query termination at this stage.
• Normalization: This stage, also known as algebrization, converts parsed SQL query to a tree of logical operators termed as “query tree”.

Compilation
Next, SQL Server attempts to compile the query.

• Optimization: SQL Server's Query Optimizer evaluates different plans to execute and chooses the least costly one. The process it uses, interestingly, isn't exhaustive — checking every possible plan would require an unrealistic amount of time for complex queries. Instead, the optimizer uses heuristic algorithms and statistical metadata from distribution statistics objects to create a reasonable plan quickly.
• Plan Generation: After the query optimization, SQL Server generates the query execution plan - the blueprint to execute the given query. These plans are stored in the "Plan Cache". If similar queries are used often, SQL Server can save resources by caching and reusing their execution plans.

Execution
With the plan in place, SQL Server moves to executing the query.

• Execution Context Generation: The Query Execution engine generates an execution context for the query, a set of instructions that execute in line with the generated plan.
• Data Retrieval: The system then undertakes activities like opening file handles, memory allocation based on the generated steps. Pages with necessary records are loaded from the disk into the buffer, if they aren't already there.
• Data Return: After all processing steps are carried out; the Server retrieves data according to the instructions and sends it back to the client who requested it.

Conclusion

SQL query processing is a complex process that involves several steps, from parsing to execution. SQL Server is tuned using strategies like cost-based query optimization and execution plan caching to manage this operation as fast and effectively as possible. When working with SQL Server in real-world applications, knowing the intricacies of this process can help tremendously with diagnostics and performance optimization.

Recall that a significant portion of SQL Server's performance is dependent on elements other than the query processing phase. These include the appropriate use of indexes, current statistics, proper database design, well-structured queries, suitable hardware, and routine maintenance. But the first step to becoming an expert with this potent relational database engine is to have a comprehensive grasp of the process that SQL Server goes through, from accepting a request to producing a response.

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