How MySQL Chooses the Cheapest Index for COUNT(*) and When It Gets It Wrong

This article examines whether SELECT COUNT(*) causes full‑table scans, explains MySQL’s optimizer cost‑based index selection (including IO and CPU costs), demonstrates with a 100k‑row table how auxiliary indexes are chosen, and shows cases where the optimizer’s estimates mislead performance.

Index CostOptimizer_trace_count

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How MySQL Chooses the Cheapest Index for COUNT(*) and When It Gets It Wrong

ITPUB

Jul 18, 2017 · Databases

Mastering MySQL B-Tree Indexes: How They Work and How to Optimize Queries

This article explains the inner workings of MySQL B‑Tree indexes, how the optimizer chooses the cheapest execution plan, demonstrates practical examples of index matching, cost calculation, and EXPLAIN analysis, and provides step‑by‑step troubleshooting techniques for real‑world performance issues.

B-Tree IndexDatabase PerformanceIndex Cost

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Mastering MySQL B-Tree Indexes: How They Work and How to Optimize Queries