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
SET QUOTED_IDENTIFIER must be
ON when you are creating or changing indexes on computed columns or indexed views. For more information, see SET QUOTED_IDENTIFIER (Transact-SQL).
All function references in the computed column must have the same owner as the table.
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 all of the following are 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 Create indexes on persisted computed columns late in this article.
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
AVGdepend 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.
CAST and CONVERT
When you refer to string literals of the date data type in indexed computed columns in SQL Server, we recommend that you explicitly convert the literal to the date type that you want by using a deterministic date format style. For a list of the date format styles that are deterministic, see CAST and CONVERT.
For more information, see Nondeterministic conversion of literal date strings into DATE values.
Implicit conversion of non-Unicode character data between collations is considered nondeterministic, unless the compatibility level is set to
80 or earlier.
When the database compatibility level setting is
90, you can't 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
DATEFORMAT settings are consistent in your databases and applications.
90 corresponds to SQL Server 2005 (9.x).
The computed_column_expression must be precise. A computed_column_expression is precise when one or more of the following is true:
It isn't an expression of the float or real data types.
It doesn't use a float or real data type in its definition. For example, in the following statement, column
yis 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);
Any float or real expression is considered imprecise and cannot be a key of an index; a float or real expression can be used in an indexed view but not as a key. This is true also for computed columns. Any function, expression, or user-defined function is considered imprecise if it contains any float or real expressions. This includes logical ones (comparisons).
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 can't 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
ANSI_NULLSconnection-level option must be set to
ALTER TABLEstatement that defines the computed column is executed. The OBJECTPROPERTY function reports whether the option is on through the
The connection on which the index is created, and all connections trying
DELETEstatements that will change values in the index, must have six
SEToptions set to
ONand one option set to
OFF. The optimizer ignores an index on a computed column for any
SELECTstatement executed by a connection that doesn't have these same option settings.
NUMERIC_ROUNDABORToption must be set to
OFF, and the following options must be set to
ON implicitly sets
ON when the database compatibility level is set to
90 or higher.
Create indexes on persisted computed columns
Sometimes you can create a computed column that is defined with an expression that is deterministic yet imprecise. You can do this when 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 can't 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.
You can't create a filtered index on a computed column.