SQL Server 2008 UPDATE Statement WHERE clause precedence - sql

Question

I wrote the following query:
UPDATE king_in
SET IN_PNSN_ALL_TP_CNTRCT_CD = IN_PNSN_ALL_TP_CNTRCT_CD + '3'
WHERE COALESCE(IN_PNSN_ALL_TP_CNTRCT_TX, '') <> ''
AND CHARINDEX('3', IN_PNSN_ALL_TP_CNTRCT_CD) = 0
It checks to see if a field has a value in it and if it does it puts a 3 in a corresponding field if there isn't a 3 already in it. When I ran it, I got a string or binary data will be truncated error. The field is a VARCHAR(3) and there are rows in the table that already have 3 characters in them but the rows that I was actually doing the updating on via the WHERE filter had a MAX LEN of 2 so I was completely baffled as to why SQL Server was throwing me the truncation error. So I changed my UPDATE statement to:
UPDATE king_in
SET IN_PNSN_ALL_TP_CNTRCT_CD = k.IN_PNSN_ALL_TP_CNTRCT_CD + '3'
FROM king_in k
INNER JOIN
(
SELECT ki.row_key,
in_sqnc_nb
FROM king_in ki
INNER JOIN King_Ma km
ON ki.Row_Key = km.Row_Key
INNER JOIN King_Recs kr
ON km.doc_loc_nb = kr.ACK_ID
WHERE CHARINDEX('3', IN_PNSN_ALL_TP_CNTRCT_CD) = 0
AND COALESCE(IN_PNSN_ALL_TP_CNTRCT_TX, '') <> ''
) a
ON k.Row_Key = a.Row_Key
AND k.in_sqnc_nb = a.insr_sqnc_nb
and it works fine without error.
So it appears based on this that when doing an UPDATE statement without a FROM clause that SQL Server internally goes through and runs the SET statement before it filters the records based on the WHERE clause. Thats why I was getting the truncation error, because even though the records I wanted to update were less than 3 characters, there were rows in the table that had 3 characters in that field and when it couldn't add a '3' to the end of one of those rows, it threw the error.
So after all of that, I've got a handful of questions.
1) Why? Is there a specific DBMS reason that SQL Server wouldn't filter the result set before applying the SET statement?
2) Is this just a known thing about SQL that I never learned along the way?
3) Is there a setting in SQL Server to change this behavior?
Thanks in advance.

Answer 1

1 - Likely because your criteria are not SARGable - that is, they can't use an index. If the query optimizer determines it's faster to do a table scan, it'll go ahead and run on all the rows. This is especially likely when you filter on a function applied to the field like you do here.
2 - Yes. The optimizer will do what it thinks it best. You can get around this somewhat by using parentheses to force an evaluation order of your WHERE clause but in your example I don't think it would help since it forces a table scan regardless.
3 - No, you need to alter your data or your logic to allow indexes to be used. If you really really need to filter on existence of a certain character in a field, it probably should be it's own column and/or you should normalize that particular bit of data better.
A workaround for your particular instance would be to add a WHERE LEN(IN_PNSN_ALL_TP_CNTRCT_CD) < 3 as well.