SQL Computed Column used in queries causing performance issues - sql

Question

I have a table with columns A,B,C and another table with column username.
In column C i have a function getName(A).
getName(A) is roughly
CREATE FUNCTION [dbo].[GetName] (
#name VARCHAR(100)
)
RETURNS VARCHAR(100)
WITH SCHEMABINDING
AS
BEGIN
DECLARE #retval VARCHAR(100);
DECLARE #nextWord VARCHAR(100);
SET #retval = #name
IF EXISTS (Select 1 from someTable where username = SUSER_NAME())
BEGIN
SET #name = Replace(Replace(Replace(RTRIM(LTRIM(#name)),',',' ,'),'(','( '),')',' )')
SET #retval = LEFT(#name, 1);
WHILE CHARINDEX(' ', #name, 1) > 0
BEGIN
SET #name = LTRIM(RIGHT(#name, LEN(#name) - CHARINDEX(' ', #name, 1)));
IF CHARINDEX(' ', #name, 1) > 0
BEGIN
SET #nextWord = LTRIM(LEFT(#name, CHARINDEX(' ', #name, 1) - 1))
END
ELSE
BEGIN
SET #nextWord = #name
END
SET #retval += ' ' + CASE
WHEN #nextWord IN (
'List'
,'Of'
,'Different'
,'Words'
)
THEN #nextWord
WHEN ISNUMERIC(#nextWord) = 1
THEN #nextWord
WHEN ISDATE(#nextWord) = 1
THEN #nextWord
ELSE LEFT(#nextWord, 1)
END
END
END
RETURN #retval;
END
Now when I try to use column C in queries it basically times out. Trying to figure out if there is a way to make it faster. If the computed function for C is just referencing A it runs normal. but when its either choose A or choose the first letter of each word in A along with words in the allowed list it goes slow. If I make this function true always it goes relatively quick. I tried with the exists and still it is not fast.
Any advice would be greatly appreciated.
EDIT: I updated the function above. I should note, when the EXISTS query returns True it runs quickly, when it returns false it runs slow. That is the bigger dilemma that I am confused about.

Answer 1

This is, alas, a very reasonable function, because it is the only way to create a computed column that references another table.
The following code is safer:
BEGIN
DECLARE #retval VARCHAR(100);
IF EXISTS (SELECT 1 FROM someTable WHERE username = SUSER_NAME)
BEGIN
SET #retval = LEFT(#name, 1);
END
ELSE SET #retval = #name;
RETURN #retval;
END
The isnull() method is clever, but the original code would generate an error if there were multiple rows in the table that matched the where condition. Also, it requires considering all values in the table, rather than just the first. EXISTS knows to stop at the first matching row.
You want an index on sometable(username). You can do this either by creating a unique constraint or by creating the index explicitly.