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When a select uses scalar or multi-statement UDFs under READ_COMMITTED_SNAPSHOT, we might not get consistent results as of the time our select began - I will provide simple repro scripts.
At the time of this writing MSDN clearly states the following: ''Read committed isolation using row versioning provides statement-level read consistency''. ...
Some T-SQL code is written under the assumption that either
a TRY block successfully completes or a CATCH block is invoked. Most likely,
this is the case. However, there is a third, although rare, possibility – the
TRY block may fail, and the CATCH one is bypassed. Let me provide some
examples. I do not intend to provide a comprehensive list ...
Although there are many discussions about which kind of cursor or
loop performs the best, there is no doubt which loops perform the worst – the infinite
ones of course. Whenever you write a loop, you need to make sure that it never
runs infinitely. I will provide two examples, two rather common scenarios that
may potentially cause loops to ...
I have written up two examples when a SET ROWCOUNT command
breaks a seemingly working stored procedure or trigger. Note that currently the
best practice is to use TOP clause instead of SET ROWCOUNT, which is
in SQL Server 2008.
However, even if you never use SET ROWCOUNT yourself, some legacy code can still use it – ...
I have been posting examples of defensive database programming
for some time now. I am by no means done with this topic, there is much more to
it. Yet this time I would like to skip concrete examples and write up a long
Defensive programming is an approach that is designed to ensure
high quality of ...
Comparing SET vs. SELECT is a very popular topic, and much
of what I have to say has been said before. Assigning multiple values via
SELECT performs better, and you don’t have to repeat your code several times,
as described by Tony Rogerson here and here. However, if you have ambiguities,
SET will raise an error, but SELECT will not detect ...
Suppose that you need to implement the following logic:
Update the row
Insert a new row.
If you already are on 2008, you should use MERGE command,
and you don’t need to read this post. Prior to 2008 this logic has to be
implemented using UPDATE and INSERT commands. I will stress test several ...
It is well known that UPDATE ... FROM command does not
detect ambiguities. Also it well known that ANSI standard UPDATE may perform very
poorly and may be difficult to maintain, because it does not adhere to the
fundamental DRY (Don’t Repeat Yourself) principle. I will tweak UPDATE ... FROM
in three different ways so that you can detect or ...
The following pattern is quite common in database
IF EXISTS(some query) BEGIN
When such code runs in high concurrency situations, it may
not work as expected. I will provide a repro when such logic fails 40% of the
time. The following script provides a test table and attempts to ...
In Transact SQL you can assign a 10-character value to a
VARCHAR(5) variable, as follows:
DECLARE @c VARCHAR(5); SET @c='1234567890'; PRINT @c
The value will be silently truncated without raising an
error. Because of this behavior it may be very easy to make mistakes. For
example, consider the following table ...