SQL Performance Tuning and Optimization – SQL Interview Questions and Answers

Q1: What is query optimization, and why is it important?
Answer:
Query optimization is the process of improving the performance of SQL queries by minimizing resource usage, such as CPU and memory. It is important because inefficient queries can lead to slow performance, especially when handling large datasets.

Q2: What is an index, and how does it improve SQL query performance?
Answer:
An index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional space and slower writes. Indexes can drastically reduce query times for large tables by providing faster access to rows based on indexed columns.

Q3: What are some common methods for improving SQL query performance?
Answer:

• Use appropriate indexes: Ensure that frequently queried columns are indexed.

• Avoid SELECT *: Only retrieve the columns you need to minimize data transfer.

• Use EXPLAIN or query plans: Analyze query plans to identify bottlenecks.

• Limit the use of subqueries: Subqueries can be less efficient than joins in some cases.

Q4: How does the EXPLAIN keyword work in SQL?
Answer:
The EXPLAIN keyword is used to get the query execution plan in SQL. It shows how the database engine executes a query, including the order of table scans, joins, and the use of indexes. Analyzing this plan helps identify potential inefficiencies and areas for improvement.

Q5: What are the differences between a clustered and a non-clustered index?
Answer:

• Clustered Index: Determines the physical order of data in the table. Each table can have only one clustered index.

• Non-clustered Index: A separate structure that stores pointers to the actual data. A table can have multiple non-clustered indexes.

Q6: What is a database query plan, and why is it important for performance tuning?
Answer:
A database query plan is an execution strategy that the SQL engine follows to retrieve data. It details operations such as joins, table scans, and index usage. Analyzing the query plan helps identify inefficiencies in query execution, such as unnecessary full table scans or missing indexes.

Q7: What are the key differences between INNER JOIN and OUTER JOIN in terms of performance?
Answer:
INNER JOIN typically performs faster because it only returns rows with matching data from both tables. In contrast, OUTER JOIN (such as LEFT JOIN) requires additional processing to return unmatched rows, leading to slower performance due to the extra handling of NULL values.

Q8: How does SQL Server’s Query Optimizer work?
Answer:
SQL Server’s Query Optimizer evaluates various execution plans for a query and selects the one with the lowest estimated cost. It analyzes factors such as available indexes, statistics, and query complexity to determine the most efficient way to execute the query.

Q9: What is query caching, and how can it impact performance?
Answer:
Query caching stores the result of a query in memory so that subsequent executions of the same query can retrieve the results without re-executing the entire process. While this speeds up repetitive queries, it can sometimes lead to stale data if the underlying data changes.

Q10: How can you prevent SQL injection attacks?
Answer:
To prevent SQL injection attacks, use:

• Prepared Statements: Bind parameters to queries instead of concatenating user input.

• Stored Procedures: Use precompiled SQL statements stored in the database.

• Input Validation: Sanitize user input to ensure it only contains valid characters.