Function-based indexes contain one or more expressions as index keys.You can create indexes on functions and expressions directly.
Like computed columns, function-based indexes are helpful for user-defined ordering and DSS (Decision Support System) applications, which frequently require intensive data manipulation. Function-based indexes simplify the tasks in these applications and improve performance.
For more information on user-defined ordering, see “Using computed columns”.
For more information on computed columns see “Computed columns—overview”.
Function-based indexes are similar to computed columns in that they both allow you to create indexes on expressions.
However, there are significant differences:
A functional index allows you to index the expression directly. It does not first create the column.
A function-based index must be deterministic and cannot reference global variables, unlike a computed column.
You can create a clustered computed column index, but not a clustered function-based index.
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