Query Optimization

Cost-Based Optimization

Modern relational DBs use cost-based optimizers (CBO): generate plan candidates -> estimate cost via statistics (row counts, distributions, selectivity) -> select lowest I/O/CPU/memory plan. Stale statistics lead to suboptimal plans.

Execution Plan Analysis

Validate optimization with EXPLAIN before and after changes.

-- PostgreSQL (add ANALYZE for actual runtime stats)
EXPLAIN ANALYZE SELECT order_id, total FROM orders WHERE customer_id = 12345;
-- MySQL: EXPLAIN FORMAT=JSON ... | SQL Server: SET STATISTICS IO ON

Key indicators: Seq Scan/Table Scan = missing index | Index Scan/Seek = efficient | Hash Join = large equality joins | Nested Loop = small/indexed inner | Merge Join = pre-sorted inputs | Sort = watch disk spills

Indexing Strategies