Speed Up Your MySQL Queries: Indexing the Date Part of a DATETIME Field

Using a Separate Date Column:

• Create an index on the new DATE column.
• Use a TRIGGER or update logic to automatically populate this column with the date portion of the DATETIME field upon insertion or update.
• Add a new column to your table with a DATE data type.

Example:

CREATE TABLE my_table (
  id INT PRIMARY KEY,
  datetime_field DATETIME NOT NULL,
  date_part DATE AS (DATE(datetime_field)) NOT NULL
);

ALTER TABLE my_table ADD INDEX (date_part);

Using a Functional Index (MySQL 5.7.6 or Later):

• Create a functional index directly on the DATE(datetime_field) expression.

CREATE TABLE my_table (
  id INT PRIMARY KEY,
  datetime_field DATETIME NOT NULL
);

ALTER TABLE my_table ADD INDEX (DATE(datetime_field));

Important Note: While the second approach seems more concise, it has limitations:

• Not all query optimizers can effectively utilize functional indexes.
• Functional indexes can be less efficient than regular indexes, especially for large datasets.

Related Issues and Solutions:

• Performance impact: Indexing adds storage overhead and impacts write performance. Evaluate the trade-off between query speed and storage/write performance before creating an index.
• Queries using functions on the date part: If your queries involve functions like YEAR, MONTH, etc., consider creating separate columns for those specific parts.