Evaluate Nested Query Performance with Detailed Metrics

Analyzing Subquery Performance with EXPLAIN ANALYZE

Write a PostgreSQL query to analyze a query that includes a subquery using EXPLAIN ANALYZE.

Solution:

-- Analyze the performance of a query with a subquery filtering employee IDs.
EXPLAIN ANALYZE 
SELECT name, salary 
FROM Employees 
WHERE department IN (
    SELECT department 
    FROM Departments 
    WHERE location = 'New York'
);

Explanation:

• Purpose of the Query:
• To examine how PostgreSQL executes a query with a subquery in the WHERE clause.
• Helps in understanding the impact of nested queries on performance.
• Key Components:
• IN (SELECT ... FROM Departments WHERE location = 'New York') : The subquery filtering departments.
• EXPLAIN ANALYZE : Displays both the planned and actual performance metrics.
• Real-World Application:
• Valuable for queries that combine data from multiple related tables using subqueries.

Notes:

• Subqueries can sometimes be rewritten as JOINs for performance improvements; the plan can indicate if a rewrite might help.

For more Practice: Solve these Related Problems:

• Write a PostgreSQL query using EXPLAIN ANALYZE to evaluate a query with a subquery in the SELECT clause retrieving a scalar value from the "Departments" table.
• Write a PostgreSQL query using EXPLAIN ANALYZE to analyze a query that uses a subquery in the WHERE clause to filter "Orders" based on "customer_id" from the "Customers" table.
• Write a PostgreSQL query using EXPLAIN ANALYZE to assess the performance of a correlated subquery in the "Sales" table.
• Write a PostgreSQL query using EXPLAIN ANALYZE to analyze a subquery that computes a maximum value used in the main query filtering the "Products" table.

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