SQL Server: Inline Table-Value UDF vs. Inline View

Question

I'm working with a medical-record system that stores data in a construct that resembles a spreadsheet--date/time in column headers, measurements (e.g. physician name, Rh, blood type) in first column of each row, and a value in the intersecting cell. Reports that are based on this construct often require 10 or more of these measures to be displayed.

For reporting purposes, the dataset needs to have one row for each patient, the date/time the measurement was taken, and a column for each measurement. In essence, one needs to pivot the construct by 90 degrees.

At one point, I actually used SQL Server's PIVOT functionality to do just that. For a variety of reasons, it became apparent that this approach wouldn't work. I decided that I would use an inline view (IV) to massage the data into the desired format. The simplified query resembles:

SELECT patient_id, 
       datetime, 
       m1.value AS physician_name, 
       m2.value AS blood_type, 
       m3.value AS rh
  FROM patient_table
INNER JOIN ( complex query here
              WHERE measure_id=1) m1...
INNER JOIN (complex query here
              WHERE measure_id=2) m2...
LEFT OUTER JOIN (complex query here
                 WHERE measure_id=3) m3...

As you can see, in some cases these IVs are used to restrict the resulting dataset (INNER JOIN), in other cases they do not restrict the dataset (LEFT OUTER JOIN). However, the 'complex query' part is essentially the same for each of these measure, except for the difference in measure_id. While this approach works, it leads to fairly large SQL statements, limits reuse, and exposes the query to errors.

My thought was to replace the 'complex query' and WHERE clause with a Inline Table-Value UDF. This would simplify the queries quite a bit, reduce errors, and increase code reuse. The only question on my mind is performance. Will the UDF approach lead to significant decreases in performance? Might it improve matters?

Thanks for your time and consideration.

Answer 1

A correctly defined TVF will not introduce any problem. You'll find many claims on the interned blasting TVFs for performance problems as compared to views or temp tables and variables. What is usualy not understood is that a TVF behaves differently from a view. A View definition is placed into the original query and then the optimizer wil rearrange the query tree as it sees fit (unless the NOEXPAND clause is used on indexed views). A TVF has different semantics and sometimes, specially when updating data, this results in the TVF output being spooled for haloween protection. It helps to mark the function WITH SCHEMABINDING, see Improving query plans with the SCHEMABINDING option on T-SQL UDFs.

Alsois important to understant the concepts of deterministic and precise function. Although they apply mostly to scalar value funcitons, TVFs can be also affected. See User-Defined Function Design Guidelines.

Answer 2

Since you need a SQL String and may not have the ability to add a view or UDF to the system, you may want to use WITH ... AS to limit the complex query to one place (At least for this statement.).

WITH complex(patientid, datetime, measure_id, value) AS
(Select... Complex Query)
SELECT patient_id
,        datetime
,        m1.value AS physician_name
,        m2.value AS blood_type
,        m3.value AS rh  
FROM patient_table
INNER JOIN (Select ,,,, From complex WHERE measure_id=1) m1...
INNER JOIN (Select ,,,, From complex WHERE measure_id=2) m2...
LEFT OUTER JOIN (Select ,,,, From complex WHERE measure_id=3) m3...