Query Plan Literacy for Java Engineers

Learn to read and optimize database query execution plans by understanding access paths (sequential vs index scans), join strategies (nested loop, hash, merge), and cardinality estimates. Using a commerce database example with PostgreSQL, the guide demonstrates how to identify performance bottlenecks like the nested-loop trap, fix them with composite indexes, ensure predicates are SARGable (index-friendly), and understand when low-selectivity filters make sequential scans preferable. The key is comparing estimated vs actual row counts in EXPLAIN ANALYZE output to align what the database does with what you intend.