Recently at work we had a need to bypass a trigger on a key
table. That got me thinking that it was worth discussing as there are quite a
few alternatives. Most triggers are conditional based on the data itself. If the
where statement matches, something happens. Not quite as common is to do (or not
do) something based on external information - who the user is or what computer
issued the statement or maybe something even more complicated.
Before we begin, we haven't had a good puzzle/contest lately, so here goes!
Now, back to our regularly scheduled article.... The easiest option is to disable the trigger. If you haven't done that
before, it's easy to do, just issue the following statement:
alter table tablename disable trigger triggername
And then do this to turn it back on:
alter table tablename enable trigger triggername
There are two downsides to this approach.
- The first and most critical is that you're disabling trigger execution
for all users. Unless you can guarantee that no other users will make
changes (the best method to guarantee that is to put the database in single
user mode, less intrusive but less effective would be to temporarily deny
access to all logins) you run the risk of having one or more changes bypass
the trigger logic. Not good.
- The second is that if the table is replicated, it just won't work. SQL
won't let you modify a replicated table. It will let you drop the
trigger and put it back when you're done. It is possible to monkey
with the system tables so that you can do the alter, but it hardly seems
worth the effort. Dropping the trigger is ok other than the problems already
mentioned, with the additional concern that adding it back may break things
if you've configured the first/last trigger execution and this was one of
A better approach is to put logic into the trigger that allows you to control
the execution. The best way is to use something that is independent of the data
in the table - username(), app_name(), and host_name() all come to mind. The
trigger code might then look something like this:
create trigger test on dbo.customers for update as set nocount on if app_name='MyAppName' begin --real trigger code here end
Given that your application sets the connection string to include AppName=MyAppName,
the trigger will execute normally for users, but if anyone connects with any
other application the main trigger code will not be executed. Sometimes that is
what you want, sometimes you may need to fine tune it, perhaps including
host_name and/or user_name() in the condition.
Think through your condition carefully. For example, in the scenario we were
working on there was a job that ran once a day to update rows matching our
criteria. In only that case did we want to bypass our code. We had other jobs
running as well, so that ruled out host, appname, and username - all were
running from the server under the agent account. That meant we needed a
Note: We could have changed the job type to ActiveX and opened a new
connection, that would allowed us to change the appname. For this article I want
to illustrate some other alternatives that don't rely on those variables.
One common technique I use is to base actions on SPID from sysprocesses.
Every connection will have a unqiue spid and you can retrieve the value by doing
'select @@spid'. Of course you can't count on the spid being the same every time
so you have to have a way to configure your code. One way is like this:
create table TableName_Config (SPID int, BypassTrigger bit default 0)
Then when you want to bypass, you do this before you execute the code that
would cause the trigger to execute:
insert into TableName_Config (SPID, BypassTrigger) values (@@SPID, 1)
And the trigger code looks like this:
create trigger test on dbo.customers for update as declare @Test bit set nocount on select @Test = BypassTrigger from dbo.TableName_Config where spid=@@SPID if Test<>1 begin --real trigger code here end
Not bad, right? You can execute the insert manually or based on some more
complicated condition in the calling code to set the behavior. Basically we just
need a flag, how simple or complex the code to set it is elsewhere, we keep the
trigger simple. Remember that you need to clean this up afterward and/or do a
delete/insert pair each time or you run the risk of the setting getting carried
over to another user that ends up with the same spid later on.
That brings us to a simplification of that technique. Do we really need a
table so we can look up a value? What we really need is a variable we can set
with the true/false value. We can't pass a variable in to the trigger, so it has
to be set before the trigger executes. Is there a way to do that?
There is a relatively unknown set statement that will let you just that -
context_info. It let's you associate up to 128 bytes of binary data with a spid.
Instead of pushing a row into a table, you can set a variable that is only good
for the lifetime of the connection, nothing to clean up. To make it work, you do
something like this:
set context_info 0x1 (this is to bypass)
Then the trigger looks like this:
create trigger test on dbo.customers for update as declare @Test varbinary(128) set nocount on select @Test = Context_INfo from master.dbo.sysprocesses where spid=@@SPID if Test<>0x1 begin --real trigger code here end
To save you a trip, here are the highlights from Books Online:
- Session context information is stored in the context_info
column in the master.dbo.sysprocesses table. This is a
- SET CONTEXT_INFO cannot be specified in a user-defined function. You
cannot supply a null value to SET CONTEXT_INFO because the
sysprocesses table does not allow null values.
- SET CONTEXT_INFO does not accept expressions other than constants or
variable names. To set the context information to the result of a
function call, you must first place the function call result in a
binary or varbinary variable.
- When you issue SET CONTEXT_INFO in a stored procedure or trigger,
unlike in other SET statements, the new value set for the context
information persists after the stored procedure or trigger completes.
How's that for a few different techniques? Have you used any of these or
maybe have another method you prefer? Add a comment, I usually reply the