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From Kristian Waagan <Kristian.Waa...@Sun.COM>
Subject Re: Access to old and new values of the triggering table's columns inside the trigger action when trigger action is a stored procedure...
Date Sun, 07 Feb 2010 11:15:52 GMT
Knut Anders Hatlen wrote:
> Mamta Satoor <msatoor@gmail.com> writes:
>
>   
>> Rick, thanks as always. All this makes sense. My recent emails about
>> trigger behaviors are to see where we can avoid having before and
>> after copies of the LOB columns in the triggering table so we don't
>> run into OOM when it can be avoided.
>>
>> Since the only way to pass before and old values to the stored
>> procedure in trigger action is through formal arguments, then I guess
>> we can safely assume that before and after values of LOB columns from
>> triggering tables will never make their way into stored procedure. The
>> Derby Reference manual says "Note: Data-types such as BLOB, CLOB, LONG
>> VARCHAR, LONG VARCHAR FOR BIT DATA, and XML are not allowed as
>> parameters in a CREATE PROCEDURE statement."
>>     
>
> This limitation is likely to be lifted, though. See DERBY-4066.
>
>   
>> So based on the fact that stored procedure gets before and old values
>> through parameters and LOBs can't be passed as parameters to stored
>> procedure, there is no need for us to keep before and after values of
>> LOB columns from the triggering table when the trigger action is a
>> stored procedure.
>>     
>
> I haven't looked at the code, so I don't know how viable the different
> approaches are, but I would think that looking at which columns the
> different triggers actually reference is a more robust way to determine
> which columns to copy into the before and after images. Then we would
> save memory/copying for trigger actions that don't call stored
> procedures too, and also for non-LOB types.
>
>   
I agree with Knut Anders on this one, it would be nice if we could 
determine which columns to copy.

That said, as part of my current LOB work, I'm looking at how to avoid 
materializing (or objectifying) LOB columns for triggers. This will 
definitely help for the cases where the LOB columns aren't referenced / 
used, and it may also enable us to keep the values as streams in other 
cases where the LOB is actually used in the trigger.
This work is still in the early phase, but I have been able to remove 
some of the work-arounds / fixes put in place earlier. It is not yet 
clear to me if this approach will get me anywhere, but I'll investigate 
further.


Regards,
-- 
Kristian


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