SQL Performance Tuning: Real Examples

Analyze and Optimize a Slow Query

Given a slow query and its execution plan, identify bottlenecks and suggest improvements.

Expected output: A list of optimizations such as adding indexes or rewriting the query.

Hint: Look for full table scans and missing indexes.

Rewrite Subqueries as JOINs

Convert provided subquery examples into JOIN statements and compare performance.

Expected output: Optimized queries using JOINs with improved execution times.

Hint: Focus on correlated subqueries that can be replaced by JOINs.

An execution plan shows how the database engine executes a query, detailing operations like scans and joins. It helps identify performance bottlenecks and guides optimization.

Indexes allow the database to quickly locate rows matching query conditions without scanning the entire table, significantly reducing query execution time.

SELECT * retrieves all columns, which may include unnecessary data, increasing I/O and memory usage, thus slowing down query performance.