Avoid calling a scalar function multiple times in an UPDATE statement

Question

Is there any way to modify the following UPDATE statement so that the scalar function is called only once, not twice?

UPDATE a
    SET SomeField = b.SomeField
    FROM TableA AS a
    JOIN TableB b ON b.Link = a.Link
    WHERE b.CostMin <= @Cost * dbo.OneScalarFunction(@CurrencyFrom, b.Currency)
        AND b.CostMax >= @Cost * dbo.OneScalarFunction(@CurrencyFrom, b.Currency)

P.S. Using the BETWEEN operator does not help - SQL Server calls the scalar function twice anyway.

Answer 1

This is often significantly more performant, but requires you to change the scalar function to an inline table valued function.

UPDATE
  a
SET
  SomeField = b.SomeField
FROM
  TableA AS a
CROSS APPLY
  dbo.oneInlineTableValuedFunction(@CurrencyFrom, e.Currency) AS ITVF
INNER JOIN
  TableB b
    ON b.Link = a.Link
WHERE
      b.CostMin <= @Cost * ITVF.exchangeRate
  AND b.CostMax >= @Cost * ITVF.exchangeRate

Although table valued functions return tables, you can choose to retun just one row with one field. Then you're using it effectively as a scalar function - But, you get all the benefits of how SQL Server can optimise the query above...
- If the TVF is Inline (and not multi-statement)
- The TVF gets expanded out into the query
- The result is performance dramatiaclly better than scalar functions

Example Inline Table Valued Function:

CREATE FUNCTION dbo.oneInlineTableValuedFunction (
                      @currencyFrom   VARCHAR(32),
                      @currencyTo     VARCHAR(32)
)
RETURNS TABLE
AS
RETURN (
  SELECT
    exchangeRate
  FROM
    dbo.someTable
  WHERE
        currencyFrom = @currencyFrom
    AND currencyTo   = @currencyTo
)

Deliberately trivial

One example post about this: scalar-functions-inlining-and-performance

If you seach the web for INLINE CROSS APPLY SCALAR FUNCTION PERFORMANCE I'm sure you'll get a whole lot more.

Answer 2

Try the BETWEEN operator.

WHERE functionCall(..) BETWEEN minValue AND maxValue

http://www.w3schools.com/sql/sql_between.asp

Answer 3

You could try using BETWEEN (as below), although you'd need to test it as I've got a sneaking suspicion the database might split this out to execute it as >= and <= anyway..

UPDATE a
    SET SomeField = b.SomeField
    FROM TableA AS a
    JOIN TableB b ON b.Link = a.Link
    WHERE  @Cost * dbo.OneScalarFunction(@CurrencyFrom, e.Currency) BETWEEN b.CostMin AND b.CostMax