SQL Query Optimization

Framework

IRON LAW: Measure Before Optimizing

NEVER guess which query is slow or why. Use EXPLAIN (EXPLAIN ANALYZE in
PostgreSQL) to see the actual execution plan. The database's plan often
differs from what you expect — a query you think is efficient may do
a full table scan, and a complex-looking query may use an index perfectly.

Measure → identify bottleneck → fix → measure again.

EXPLAIN Output Reading