When Do Database Indexes Hurt More Than Help?

Purpose of Indexes

Database indexes store a sorted copy of column values, enabling fast row location for operations such as JOIN, GROUP BY, WHERE and ORDER BY. They improve query performance but should be used selectively.

Common Drawbacks

Additional Storage Overhead

Indexes require extra space for the indexed values and pointers to rows. Integer columns add modest overhead, while string columns store the full value and its length. Many indexes on large tables can significantly increase storage requirements.

Write‑Operation Performance Degradation

INSERT, UPDATE and DELETE must also modify every related index, slowing write throughput. For example, inserting one million rows may take 10‑15 seconds without indexes but around 2 minutes with several indexes. High‑frequency write workloads need careful index planning.

Index Management and Cleanup

Unused indexes waste space and can confuse the optimizer. Regular audits are recommended. The following query lists potentially unused indexes (where cardinality is NULL or 0):

SELECT table_name, index_name, non_unique, seq_in_index, column_name, collation, cardinality, sub_part, packed, index_type, comment, index_comment
FROM information_schema.STATISTICS
WHERE table_schema = 'your_database_name'
  AND index_name != 'PRIMARY'
  AND (cardinality IS NULL OR cardinality = 0)
ORDER BY table_name, index_name, seq_in_index;

After confirming an index is unnecessary, drop it safely:

ALTER TABLE your_table_name DROP INDEX your_index_name;

Assessing Index Efficiency

Even used indexes may provide negligible benefit. Retrieve full index metadata to evaluate each one:

SELECT * FROM information_schema.statistics;

Testing Impact with Invisible Indexes (MySQL)

MySQL’s invisible indexes allow you to test the effect of removing an index without affecting write operations. Make an index invisible:

ALTER TABLE your_table_name ALTER INDEX your_index_name INVISIBLE;

If the index proves unnecessary, keep it invisible or drop it; otherwise, restore visibility:

ALTER TABLE your_table_name ALTER INDEX your_index_name VISIBLE;

Conclusion

Indexes boost query speed but increase storage consumption, especially on large datasets.

They can degrade write performance; high‑write scenarios require careful index selection.

Excessive or unused indexes add optimizer complexity and maintenance overhead.

Decide to add an index based on application requirements, tolerance for storage and write costs, and thorough performance testing.