A Practical Guide to Database Indexing for Analysts: When and Why It Matters

Data analyst Michael Nocito published a practical guide on August 8, 2026, explaining when and how to use database indexes to speed up slow queries. The guide focuses on the fundamental difference between a full table scan, where the engine reads every row, and an index scan, where it jumps directly to the needed value using a sorted structure. Nocito demonstrates concepts using a 500,000-row SQLite orders table, with each query timed across five runs to provide real-world performance comparisons. Readers are shown how to use EXPLAIN QUERY PLAN to diagnose whether a query is scanning an entire table, a technique that works across major databases including PostgreSQL. The guide also covers the costs and limitations of indexes, including why an index on one column offers no benefit to queries filtering on a different column.