Question

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Why does the Execution Plan include a user-defined function call for a computed column that is persisted?

I’ve added a persisted computed column to a table.

ALTER TABLE guest
  ADD FullName AS
  dbo.complicatedFunction(ISNULL(FirstName, N'') + ISNULL(LastName, '')) PERSISTED
GO

I also added a non-clustered index on that field.

When I do a select from that table and include the column into a where statement it takes very long to execute.

I tried replacing the persisted column with the expression directly and it takes the same amount of time to execute, which led me to believe that FullName field is being recalculated for every row when I try to select something from that table.

How can I avoid that recalculation and get the query to use the index?

Answer 1

Martin Smith posted a link to a great explanation on why this is happening, and after reading through it, I replaced a call to the UDF with it’s body and it solved my problem.