What is the most important factor for join performance?

Proper indexing is the most critical factor. Index all join columns (foreign keys and primary keys). This can provide 10-100x performance improvement. Without indexes, joins require full table scans which are extremely slow for large tables.

How do you analyze and optimize a slow join query?

Use EXPLAIN to analyze execution plan. Look for full table scans, missing index usage, and high cost operations. Create indexes on join columns. Consider covering indexes. Filter with WHERE to reduce rows. Update table statistics. Test with production data volumes.

Optimize this slow query: SELECT c.name, o.total FROM customers c JOIN orders o ON c.id = o.customer_id WHERE o.date >= '2024-01-01';

-- Create composite index for join and filter

CREATE INDEX idx_orders_customer_date ON orders(customer_id, date);

-- Ensure customers table has primary key index

CREATE INDEX idx_customers_pk ON customers(id);

-- Query will now use indexes efficiently

Composite index supports both join condition and WHERE filter. Primary key index optimizes customer lookups.