Creating Indexes on Computed Columns

You can define indexes on computed columns as long as the following requirements are met:

• Ownership requirements

• Determinism requirements

• Precision requirements

• Data type requirements

• SET option requirements

Ownership Requirements

All function references in the computed column must have the same owner as the table.

Determinism Requirements

Expressions are deterministic if they always return the same result for a specified set of inputs. The IsDeterministic property of the COLUMNPROPERTY function reports whether a computed_column_expression is deterministic.

• 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.

  • 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.

Precision Requirements

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 float or real data types.

• It does not use a float or real data type in its definition. For example, in the following statement, column y is int and 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 text, ntext, or image data types.

• Computed columns derived from image, ntext, text, varchar(max), nvarchar(max), varbinary(max), and xml data types can be indexed as long as the computed column data type is allowable as an index key column.

• Computed columns derived from image, ntext, and text data 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:

  • ANSI_NULLS

  • ANSI_PADDING

  • ANSI_WARNINGS

  • ARITHABORT

  • CONCAT_NULL_YIELDS_NULL

  • QUOTED_IDENTIFIER

  Setting ANSI_WARNINGS to ON implicitly sets ARITHABORT to ON when the database compatibility level is set to 90. If the database compatibility level is set to 80 or earlier, the ARITHABORT option must explicitly be set to ON. For more information, see SET Options That Affect Results.