如何避免为sum（）反复调用cpu密集函数？

Question

I have a function that performs a cpu heavy calculation on a passed id I am working on a report to summarize the results, but for performance reasons I only want to call the function once per id

Here is a simple example of the working SQL BUT it calls the function once for each column. How can I rewrite the query to only call the function once per identifier?

    SELECT 
    SUM( case myfunction(employee_id) WHEN 'bob' THEN 1 ELSE 0 END) as "bob total",
        SUM( case myfunction(employee_id) WHEN 'joe' THEN 1 ELSE 0 END) as "Joe total",
    SUM( case myfunction(employee_id) WHEN 'tom' THEN 1 ELSE 0 END) as "Toms total"
    FROM employee

Answer 1

You can just put the logic in a subquery before doing the sum, then reference the single column.

SELECT 
    SUM( case FunctionResult WHEN 'bob' THEN 1 ELSE 0 END) as "bob total",
    SUM( case FunctionResult WHEN 'joe' THEN 1 ELSE 0 END) as "Joe total",
    SUM( case FunctionResult WHEN 'tom' THEN 1 ELSE 0 END) as "Toms total"
FROM    
    (   SELECT myfunction(employee_id) AS FunctionResult
        FROM employee
    ) AS e;

A better solution in terms of performance though would be to look at what your scalar function actually does, and ask yourself does it need to be a function, can you make the solution set based?

Answer 2

Instead of FROM employee , use a derived table that is defined like this:

SELECT *, myfunction(employee_id) AS myFuncResult
FROM employee

Now, you have myFuncResult as an additional column. Let's hope the optimizer does not mess with this strategy. It does not have a good understanding of scalar function cost (in fact, none).

Answer 3

Since you are using SQL Server you could try the CROSS APPLY expression, just replace .Result with the name of the column that your function returns:

SELECT 
    SUM( case func.Result WHEN 'bob' THEN 1 ELSE 0 END) as "bob total",
    SUM( case func.Result WHEN 'joe' THEN 1 ELSE 0 END) as "Joe total",
    SUM( case func.Result WHEN 'tom' THEN 1 ELSE 0 END) as "Toms total"
FROM employee e
CROSS APPLY myfunction(e.employee_id) as func