Knut Anders Hatlen wrote:
Mamta Satoor <[email protected]> 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
