🚀 5 Advanced SQL Interview Questions on Indexing & Query Optimization! #SQL #PerformanceTuning

🚀 Enhance your SQL expertise with these advanced interview questions focusing on indexing and query optimization!

✅ 1. Query Execution Plan:

A query execution plan outlines the sequence of operations the database engine uses to execute a query. By examining the plan, you can identify performance bottlenecks and optimize query performance.
Example:

-- In SQL Server, use:

EXPLAIN QUERY PLAN
SELECT * FROM Orders WHERE OrderDate v '2025-01-01';

Analyzing the plan helps in understanding how the query is executed and where improvements can be made.

✅ 2. Sargability:

Sargability refers to a condition where a query can utilize indexes effectively, enhancing performance. Non-sargable queries, such as those applying functions to indexed columns in the WHERE clause, can lead to full table scans.

Example:

-- Non-sargable:
SELECT * FROM Orders WHERE YEAR(OrderDate) = 2025;

-- Sargable:
SELECT * FROM Orders WHERE OrderDate v= '2025-01-01' AND OrderDate v '2026-01-01';

The sargable query allows the use of indexes on OrderDate, improving performance.

✅ 3. Join Order and Query Performance:

The order in which tables are joined can significantly impact query performance. Joining smaller tables first can reduce the dataset size early, leading to faster execution.

Strategy:

Join smaller tables first: Reduces the number of rows processed in subsequent joins.

Use appropriate join types: Choose between INNER, LEFT, or RIGHT joins based on the data and requirements.

✅ 4. Covering Indexes:

A covering index includes all the columns a query needs, allowing the database to retrieve data directly from the index without accessing the table. This reduces I/O operations and speeds up query performance.

Example:

-- Creating a covering index:

CREATE INDEX idx_Covering ON Orders (CustomerID, OrderDate, TotalAmount);

This index covers queries that select CustomerID, OrderDate, and TotalAmount from the Orders table.

✅ 5. Parameter Sniffing in SQL Server:

Parameter sniffing occurs when SQL Server uses the parameter values passed during the first execution of a stored procedure to generate an execution plan, which might not be optimal for other parameter values.
Resolution Strategies:

Use OPTION (RECOMPILE): Forces SQL Server to recompile the query with the current parameter values.

Optimize for specific values: Use query hints to optimize for typical parameter values.

Use local variables: Assign parameters to local variables within the procedure to prevent parameter sniffing.

💡 Master these advanced SQL concepts to excel in your next interview!

💬 Have questions or need further clarifications? Drop your queries in the comments!

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