MariaDB & MySQL Query Performance Optimization: Index Cardinality and EXPLAIN Plans for E-Commerce

Why Adding More RAM Does Not Fix Slow Queries

When database queries degrade under load, increasing server RAM offers only temporary relief if queries perform full table scans. Understanding how the query optimizer uses index cardinality is crucial for high-performance e-commerce platforms.

Using EXPLAIN ANALYZE to Locate Bottlenecks

Execute EXPLAIN ANALYZE SELECT ... to inspect actual execution times versus estimated optimizer costs:

EXPLAIN ANALYZE
SELECT o.id, o.total, c.email 
FROM orders o 
JOIN customers c ON o.customer_id = c.id 
WHERE o.status = 'COMPLETED' 
AND o.created_at >= '2026-01-01';

Look for Filter: ... (cost=...) and ensure composite indexes exist covering both the filter predicates and join columns (e.g., INDEX idx_status_created (status, created_at)).