MySQL B-Tree Index Not Used Fix

In this tutorial, you'll learn about MySQL B. We cover key concepts, practical examples, and best practices.

MySQL is ignoring a B-Tree index and using a full table scan because the WHERE clause does not match the leading column of the composite index.

The Wrong Way

CREATE INDEX idx_status_date ON orders(status, created_at);
EXPLAIN SELECT * FROM orders ORDER BY created_at\G

Output:

type: ALL -- Full table scan
rows: 1000000
Extra: Using filesort

The Right Way

CREATE INDEX idx_created_at ON orders(created_at);
EXPLAIN SELECT * FROM orders ORDER BY created_at\G

Output:

type: index
rows: 1000000
Extra: Using index -- No filesort needed

Step-by-Step Fix

1. Check the EXPLAIN output for 'type': look for 'ALL' (full scan) vs 'ref' or 'range'

2. Ensure leading columns of composite index match the WHERE clause order

3. Avoid function calls on indexed columns: WHERE DATE(created_at) = '2025-01-01'

4. Use covering indexes to include all queried columns

5. ANALYZE TABLE to update index cardinality statistics

Prevention Tips

• Test all queries with the database's explain plan tool before deploying to production.
• Monitor query performance trends using built-in monitoring tools.
• Set up automated index usage analysis in CI/CD pipelines.
• Review database configuration quarterly against workload patterns.
• Keep database statistics up to date with regular maintenance operations.
• Use DodaTech's monitoring tools to track query performance regressions.

Common Mistakes with index btree

1. Mixing let bindings with <- bindings in do notation, producing type errors
2. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
3. Non-exhaustive pattern matches that compile with warnings then crash at runtime

These mistakes appear frequently in real-world MYSQL code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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