Indexes on Computed Columns
You can define indexes on computed columns as long as the following requirements are met:
Data type requirements
SET option requirements
All function references in the computed column must have the same owner as the table.
The computed_column_expression must be deterministic. A computed_column_expression is deterministic when one or more of the following is true:
All functions that are referenced by the expression are deterministic and precise. These functions include both user-defined and built-in functions. For more information, see Deterministic and Nondeterministic Functions. Functions might be imprecise if the computed column is PERSISTED. For more information, see Creating Indexes on Persisted Computed Columns later in this topic.
All columns that are referenced in the expression come from the table that contains the computed column.
No column reference pulls data from multiple rows. For example, aggregate functions such as SUM or AVG depend on data from multiple rows and would make a computed_column_expression nondeterministic.
The computed_column_expression has no system data access or user data access.
Any computed column that contains a common language runtime (CLR) expression must be deterministic and marked PERSISTED before the column can be indexed. CLR user-defined type expressions are allowed in computed column definitions. Computed columns whose type is a CLR user-defined type can be indexed as long as the type is comparable. For more information, see CLR User-Defined Types.
Implicit conversion of non-Unicode character data between collations is also considered nondeterministic, unless the compatibility level is set to 80 or earlier.
When the database compatibility level setting is 90, you cannot create indexes on computed columns that contain these expressions. However, existing computed columns that contain these expressions from an upgraded database are maintainable. If you use indexed computed columns that contain implicit string to date conversions, to avoid possible index corruption, make sure that the LANGUAGE and DATEFORMAT settings are consistent in your databases and applications.
The computed_column_expression must be precise. A computed_column_expression is precise when one or more of the following is true:
It is not an expression of the
It does not use a
realdata type in its definition. For example, in the following statement, column
intand deterministic but not precise.
CREATE TABLE t2 (a int, b int, c int, x float, y AS CASE x WHEN 0 THEN a WHEN 1 THEN b ELSE c END);
The IsPrecise property of the COLUMNPROPERTY function reports whether a computed_column_expression is precise.
Data Type Requirements
The computed_column_expression defined for the computed column cannot evaluate to the
Computed columns derived from
xmldata types can be indexed as long as the computed column data type is allowable as an index key column.
Computed columns derived from
textdata types can be nonkey (included) columns in a nonclustered index as long as the computed column data type is allowable as a nonkey index column.
SET Option Requirements
The ANSI_NULLS connection-level option must be set to ON when the CREATE TABLE or ALTER TABLE statement that defines the computed column is executed. The OBJECTPROPERTY function reports whether the option is on through the IsAnsiNullsOn property.
The connection on which the index is created, and all connections trying INSERT, UPDATE, or DELETE statements that will change values in the index, must have six SET options set to ON and one option set to OFF. The optimizer ignores an index on a computed column for any SELECT statement executed by a connection that does not have these same option settings.
The NUMERIC_ROUNDABORT option must be set to OFF, and the following options must be set to ON:
Setting ANSI_WARNINGS to ON implicitly sets ARITHABORT to ON when the database compatibility level is set to 90 or higher.
You can create an index on a computed column that is defined with a deterministic, but imprecise, expression if the column is marked PERSISTED in the CREATE TABLE or ALTER TABLE statement. This means that the Database Engine stores the computed values in the table, and updates them when any other columns on which the computed column depends are updated. The Database Engine uses these persisted values when it creates an index on the column, and when the index is referenced in a query. This option enables you to create an index on a computed column when Database Engine cannot prove with accuracy whether a function that returns computed column expressions, particularly a CLR function that is created in the .NET Framework, is both deterministic and precise.