Should You Index Boolean Fields? When Does It Make a Difference?

Indexing Boolean Fields for Enhanced Performance

Queries often involve filtering data based on boolean fields. Since boolean fields are typically stored as a single bit or byte, it's commonly assumed that indexing them would provide negligible performance gain. However, in certain scenarios, indexing boolean fields can significantly improve query efficiency.

When Indexing a Boolean Field Benefits Performance

Indexing a boolean field is beneficial when:

• Data distribution is highly skewed: If the majority of rows have the same boolean value (e.g., most rows have isok=0), an index can speed up queries by eliminating the need to scan a large number of rows.
• Query selectivity is low: Queries that retrieve a small percentage of the table (e.g., WHERE isok=1) benefit from boolean indexes. The index optimizes the data access, enabling the engine (such as InnoDB) to identify matching rows efficiently.

Evidence from Real-World Example

One user reported experiencing a significant performance boost by indexing a boolean field. In their case, a table with 4 million rows had approximately 1000 rows with the boolean flag set. The query with WHERE isok=1 took over 9 seconds to execute. After adding an index on the boolean field, the query time reduced to a fraction of a second.

Conclusion

While indexing boolean fields is generally not recommended, there are specific scenarios where it can enhance query performance. When data distribution is skewed, or query selectivity is low, indexing a boolean field can result in orders of magnitude improvement in query execution time.

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