Multi-Table Join Performance Tuning
Read join plans, force a join order with hints, and reduce intermediate row counts to keep multi-table queries fast.
Joins Multiply Row Counts
If A has 10k rows matching the filter and B has 5 matches per A row, A JOIN B produces 50k. Add C with 5 matches per row → 250k. Intermediate row counts drive cost.
Filter Early, Join Later
Apply selective predicates as early as possible:
-- Slow — filters AFTER joining:
SELECT u.email FROM users u JOIN orders o ON o.user_id = u.id
WHERE u.country = 'US' AND o.total > 1000;
-- Same query, planner usually pushes filters down automatically.
-- For complex queries, force it with a CTE/subquery filter.All lessons in this course
- Cross Joins and Cartesian Products
- Lateral Joins (LATERAL JOIN)
- Anti-Joins and Semi-Joins (NOT EXISTS)
- Multi-Table Join Performance Tuning