Query optimization and execution plans

Query Optimization and Execution Plans

Efficient query performance is crucial for database scalability and responsiveness. Query optimization involves refining SQL queries to reduce execution time and resource consumption.

1. Query Optimization Techniques

1.1 Indexing

Speeds up searches and joins by creating a sorted lookup table.
Types of Indexes:
Clustered Index: Sorts rows physically in the storage (one per table).
Non-clustered Index: Stores pointers to data, allowing multiple indexes per table.
Full-Text Index: Optimized for text searches (used in search engines).

Example: Creating an index on a frequently queried column.

CREATE INDEX idx_customer_email ON customers(email);

1.2 Avoiding SELECT *

Selecting unnecessary columns increases I/O load.

Instead, retrieve only required fields:

SELECT name, email FROM customers WHERE status = 'active';

1.3 Using Proper Joins

INNER JOIN is typically faster than OUTER JOIN when extra data isn’t needed.
Use EXISTS instead of IN for better performance in large datasets.

Example:

-- Better performance with EXISTS:
SELECT * FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.customer_id);

1.4 Query Rewriting and Subqueries

Avoid nested subqueries; use JOINs instead.

-- Inefficient subquery:
SELECT name FROM customers WHERE id IN (SELECT customer_id FROM orders);

-- Optimized using JOIN:
SELECT DISTINCT c.name FROM customers c
JOIN orders o ON c.id = o.customer_id;

1.5 Using Caching and Prepared Statements

Query caching reduces redundant execution.
Prepared statements reuse query execution plans.

Example in PostgreSQL:

PREPARE customer_query (int) AS
SELECT * FROM customers WHERE customer_id = $1;
EXECUTE customer_query(101);

2. Execution Plans

An execution plan describes how the database engine processes a query.

2.1 Analyzing Execution Plans

EXPLAIN (SQL command) provides insights into query execution.
EXPLAIN ANALYZE runs the query and provides actual performance stats.

Example:

EXPLAIN ANALYZE SELECT * FROM customers WHERE email = 'test@example.com';

2.2 Key Metrics in Execution Plans

Seq Scan (Sequential Scan): Table scanned row by row (slow).
Index Scan: Uses an index for efficient lookup.
Hash Join / Merge Join: Different strategies for joining tables.

Optimizing queries based on execution plans can drastically improve database performance.

Query optimization and execution plans