Triggers and Flag bit

[Sharif Islam]

I have a perl script (uses sql query) that will run if there is any
update in the row. Currently, I am a trigger that sets the DateModified
field with getdate() Then my perl script looks for certain range of
modified dates and runs the necessary query.

I am trying to come up with a different mechanism where I don't have to
use the date field. I am looking into flag bits. But not sure how
to use it.

Should I create a trigger that will set the flag bit after any update?
This way the script will just look for the updated records, regardless
of what time it was updated. Maybe I am misunderstanding flag bits, then
after another subsequent update how would the perl/sql script know which
records were updated? I hope this make sense.

thanks.

--s

[Erland Sommarskog]

Sharif Islam (mi****@spam.uiuc.edu) writes:

I have a perl script (uses sql query) that will run if there is any
update in the row. Currently, I am a trigger that sets the DateModified
field with getdate() Then my perl script looks for certain range of
modified dates and runs the necessary query.

I am trying to come up with a different mechanism where I don't have to
use the date field. I am looking into flag bits. But not sure how
to use it.

Should I create a trigger that will set the flag bit after any update?
This way the script will just look for the updated records, regardless
of what time it was updated. Maybe I am misunderstanding flag bits, then
after another subsequent update how would the perl/sql script know which
records were updated? I hope this make sense.

There are a couple of alternatives. Which version of SQL Server are you
using? If you use SQL 2005, Query Notification is an interesting
alternative. Your Perl script would issue a query, and then wait until
the result set changes. Of course, this presumes that you use an API
that supports Query Notification, but Win32::SqlServer, available from
my web site on http://www.sommarskog.se/mssqlperl/index.html does.

Another alternative is to use a timestamp column. A timestamp column
in SQL Server has nothing do with date and time, but is automatically
updated each time a row is updated with a database-unique value that
grows monotonically. Thus, the Perl script could look at the timestamp
column and save the latest value as a high-water mark. This would at
least save you the trigger.

I don't believe in flag bits. In this case, the Perl script would have
to flip them back, and that's more complex.
--
Erland Sommarskog, SQL Server MVP, es****@sommarskog.se

Books Online for SQL Server 2005 at
http://www.microsoft.com/technet/pro...ads/books.mspx
Books Online for SQL Server 2000 at
http://www.microsoft.com/sql/prodinf...ons/books.mspx

[--CELKO--]

>I am trying to come up with a different mechanism where I don't have to use the date field [sic: columns are not fields]. I am looking into flag bits. <<

All that will do is destroy data you have collected. Just use the
date.

[Sharif Islam]

Erland Sommarskog wrote:

Sharif Islam (mi****@spam.uiuc.edu) writes:

>Thanks, the timestamp solution seems feasible. I am little confused on
how to use it.

>
Rather than using @@DBTS, min_active_rowversion() is a better choice.
This function was added in SP2, and it was added to Books Online as late
in the September 2007 edition (see my signature for download link). Looking
at @@dbts can cause some issues when there are uncommitted transaction.

Thanks for the help. Here's how I am using it. I created a column 'Flag'
with timestamp datatype.

------

declare @before timestamp
declare @after timestamp
set @before= min_active_rowversion() -1
update MyTable set MyCol ='Test' where MyCol like 'Test123%'
set @after = min_active_rowversion() -1

select ID,MyCol from MyCol where Flag -1 < @after
and Flag -1 >= @before
-----
This gave me the list of record ID that was just changed. Is this the
way to use min_active_rowversion()?

--s