Balancing Performance: When to Exclude NULL from MySQL Indexes

2024-07-27

NULL represents the absence of a specific value in a database table column.
It's distinct from both empty strings and zeros (0).
It indicates that no data is present for that particular cell in the table.

Indexing in MySQL:

An index is a special data structure that helps MySQL efficiently locate data in a table.
It acts like an organized index in a book, allowing for faster lookups based on specific columns.
Indexes store values in a sorted order, similar to an alphabetical index in a book.

Do MySQL indexes include NULL values?

They are treated as distinct values within the index, meaning they are positioned at the beginning of the sorted order.
This treatment is consistent with the concept of NULL signifying the absence of a value, placing it "before" any actual values in the index.

Implications of indexing NULL values:

While including NULL in indexes allows searching for NULL values efficiently using the IS NULL operator, it can also have some drawbacks:
- Increased index size: NULL values add to the index size, potentially impacting performance for large datasets and frequent updates.
- Potential performance degradation: In certain queries, the presence of many NULL values in an index may not significantly improve lookup efficiency.

When to consider excluding NULL from indexes:

If a column has a high percentage of NULL values (e.g., often exceeding 20-30%), you might consider excluding NULL from the index to:
- Reduce index size, potentially improving performance for inserts, updates, and deletes.
- Avoid unnecessary overhead in specific query scenarios.

Careful evaluation before excluding NULL:

Evaluate query patterns: Ensure that queries that rely on searching for NULL values with IS NULL won't be adversely affected.

Alternative approaches:

Consider using separate indexes for columns with high NULL values if necessary.
Explore alternative data modeling techniques to minimize the use of NULL values where appropriate.

You can define a virtual column that excludes NULL values using a CASE statement and create an index on this virtual column:

ALTER TABLE your_table
ADD COLUMN non_null_column CASE WHEN column_with_null IS NULL THEN 0 ELSE column_with_null END;

CREATE INDEX idx_non_null_column ON your_table(non_null_column);

Remember to drop the non_null_column after creating the index if you don't need it for other purposes.

Using partial indexes (MySQL 8.0 and later):

MySQL 8.0 introduced partial indexes, allowing you to index only specific portions of a column. You can exclude NULL values using the WHERE clause in the CREATE INDEX statement:

CREATE INDEX idx_non_null_column ON your_table(column_with_null) WHERE column_with_null IS NOT NULL;

Custom logic in your application:

If the above solutions are not suitable, you can implement logic within your application to handle NULL values before querying the database. This might involve filtering them out or assigning them default values before querying.

Example code (excluding NULL before indexing):

-- Create a table with a column allowing NULL values
CREATE TABLE my_table (
  id INT PRIMARY KEY,
  name VARCHAR(255),
  age INT,
  city VARCHAR(255)
);

-- Insert some data, including NULL values
INSERT INTO my_table (name, age, city) VALUES
  ('foo', 30, 'New York'),
  ('bar', NULL, 'London'),
  ('Charlie', 25, NULL);

-- Check if the index includes NULL values (using `SHOW INDEXES`)
SHOW INDEXES FROM my_table;

-- Filter NULL values before indexing (example)
ALTER TABLE my_table ADD COLUMN non_null_age CASE WHEN age IS NULL THEN 0 ELSE age END;
CREATE INDEX idx_non_null_age ON my_table(non_null_age);

-- Check the new index (using `SHOW INDEXES`)
SHOW INDEXES FROM my_table;