In this lesson, you will learn how permitting NULL values in your columns influences the physical footprint of your indexes and alters the way MySQL and MariaDB engine logic handles data comparisons.
The Physical Cost of Nullability
In a B-Tree index, every entry needs to be stored in a consistent format. When you define a column as NULLABLE (the default in MySQL), the database engine must allocate extra space for a "null flag"—a single bit in the record header—to track whether the value is present or absent.
While a single bit sounds trivial, the cumulative effect on a massive B-Tree index is significant. Because the index structure must account for these null flags, the index size grows. Furthermore, NULL values are treated uniquely within the index tree; they are effectively "sorted" as smaller than any non-null value, which can slightly increase the overhead of index maintenance during INSERT or UPDATE operations.
Impact on Comparison Logic
Beyond storage, nullability changes your WHERE clause behavior. Remember that NULL represents an "unknown" state, not a zero or an empty string.
Consider this index on users(email):
SELECT * FROM users WHERE email = 'test@example.com';
If your column is NOT NULL, the optimizer can perform a swift range scan. However, if the column is NULLABLE, the database engine must account for the fact that NULL values cannot be compared using standard operators like = or !=. Because NULL = NULL evaluates to UNKNOWN (not TRUE), queries looking for specific values will simply ignore the NULL entries.
If you frequently filter by IS NULL, you might need to ensure your indexes are tailored to include those nulls, but be aware that many standard range scans are less efficient when the engine has to navigate a large collection of NULL nodes within the tree structure. In short, keeping columns NOT NULL with a defined default value (like an empty string or a sentinel value) often leads to a tighter, faster index than allowing the engine to manage the overhead of nullability.
Key Takeaway: Nullable columns increase index storage overhead due to metadata flags and force the query optimizer to navigate more complex comparison logic, often making NOT NULL constraints a performance optimization strategy.