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Microsoft SQL Server 2012 Performance Tuning Cookbook by Bihag Thaker, Ritesh Shah

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Enhancing performance with index on Computed Columns

Before trying to understand what "an index on a Computed Column" is, it is good to have a basic understanding of what a Computed Column is.

As per MSDN, a Computed Column is computed from an expression that can use other columns in the same table. The expression can be a non-computed column name, constant, function, or any combination of these, connected by one or more operators. The expression cannot be a subquery.

By default, a Computed Column is a virtual column and it is recalculated every time we call it, until we specify it as PERSISTED in the CREATE TABLE or ALTER TABLE commands.

If a Computed Column is defined as being PERSISTED, it stores the calculated value and those stored values are ...

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