Computed (Expression) Indexes

In creating an index, you can use the COMPUTED BY clause to specify an expression instead of one or more columns.Computed indexes are used in queries where the condition in a WHERE, ORDER BY or GROUP BY clause exactly matches the expression in the index definition.The expression in a computed index may involve several columns in the table.

Certain limits apply to indexes.

The maximum length of a key in an index is limited to a quarter of the page size.