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Posted by Erland Sommarskog on 05/28/05 11:36
(EggsAckley@Last.com) writes:
>>o Try as much as possible to avoid iterative solution, and use set-
>> based solutions.
>
> >>Please elaborate a little on what exactly you mean by
> set-based solutions vs iterative.
>
> >>Also what do you think about cursors?
A cursor is an iterative solution. Some people hear that cursors are
evil, so they go home and replace the cursor with a WHILE loop where
they do SELECT MIN from a table or somesuch. That's typiclally even
worse.
In a set-based solution you work on all data in one statement. I steal an
example from another thread, where a poster had a trigger like this:
CREATE TRIGGER [AI_CATEGORY] ON [dbo].[CATEGORY]
FOR INSERT
AS
DECLARE @TEMP_ID AS INT,
@COUNTER AS INT,
@P_ID AS INT
SELECT @TEMP_ID = CATEGORY_ID,
@COUNTER = 1,
@P_ID = PARENT_CATEGORY_ID
FROM INSERTED
IF @P_ID IS NOT NULL
BEGIN
WHILE @TEMP_ID IS NOT NULL
BEGIN
SELECT @TEMP_ID = @P_ID,
@COUNTER = @COUNTER + 1
FROM INSERTED
END
END
UPDATE CATEGORY
SET DEPTH = @COUNTER
This is an example of an iterative solution. Here is my rewrite of this
into a set-based solution:
CREATE TRIGGER category_tri ON CATEGORY FOR INSERT AS
UPDATE c
SET DEPTH = coalesce(p.DEPTH, 0) + 1
FROM CATEGORY c
JOIN inserted i ON c.CATEGORY_ID = i.CATEGORY_ID
LEFT JOIN CATEGORY p ON i.PARENT_CATEGORY_ID = p.CATEGORY_ID
In this case, we cannot compare performance, as the iterative trigger
was incorrect, but it illustrates the two different approaches.
There are situations where iterative solutions are required, or at least
can be justified. But in many situations, there are magnitudes of
performance to gain by using a set-based solution.
--
Erland Sommarskog, SQL Server MVP, esquel@sommarskog.se
Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techinfo/productdoc/2000/books.asp
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