MySQL Hash Index Only Used With = Fix

In this tutorial, you'll learn about MySQL Hash Index Only Used With = Fix. We cover key concepts, practical examples, and best practices.

A HASH index is not being used for range queries because HASH indexes only support equality comparisons, not range scans.

The Wrong Way

CREATE INDEX idx_hash ON users(email) USING HASH;
EXPLAIN SELECT * FROM users WHERE email > 'a@example.com'\G

Output:

type: ALL -- Full table scan
possible_keys: NULL

The Right Way

CREATE INDEX idx_btree ON users(email) USING BTREE;
EXPLAIN SELECT * FROM users WHERE email > 'a@example.com'\G

Output:

type: range -- Uses B-Tree index for range scan
key: idx_btree

Step-by-Step Fix

1. Understand that HASH indexes only support = and <=> (NULL-safe equality) queries

2. Use BTREE indexes for range queries (>, <, BETWEEN, LIKE without leading wildcard)

3. HASH indexes are typically only useful with the MEMORY storage engine

4. InnoDB automatically creates B-Tree indexes even when HASH is specified

5. Use EXPLAIN to verify which index type the optimizer chooses

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 hash

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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