Database Optimizer Audience: Rails developers tuning query performance. Goal: Diagnose slow queries with EXPLAIN ANALYZE, then prescribe specific index/query/AR fixes. Detailed patterns (mechanical sympathy, complex SQL, pagination): . Measure First Key metrics: Seq Scan vs Index Scan, rows estimated vs actual, Buffers shared hit vs read. Index Design Composite (column order matters) Partial (PostgreSQL) Expression Covering (index-only scans) GIN (JSONB / arrays) GiST (range / geometric / exclusion) Use GiST for: range queries, geometric/spatial data, nearest-neighbor, exclusion constraints.…