Re: trigger with cascade delete problem

[Rick Hillegas]

On 6/16/14 1:07 AM, Tim Dudgeon wrote:
On 13/06/2014 13:59, Rick Hillegas wrote:
Maybe
you can share a simplified problem which shows why this aggregation is
so tricky.

Thanks,
-Rick



Hi Rick

Thanks for your patience!
I've tried to create a minimal version that describes the problem:

create table XXX (
    XXX_ID int primary key generated always as identity
);

create table GROUPING (
    GROUPING_ID int primary key generated always as identity,
    XXX_ID INT,
    constraint GROUPING2XXX foreign key (XXX_ID) references XXX 
(XXX_ID) on delete cascade
);

create table MAIN (
    MAIN_ID INT generated always as identity,
    GROUPING_ID INT,
    YYY_ID INT,
    NAME VARCHAR(50),
    constraint MAIN2GROUPING foreign key (GROUPING_ID) references 
GROUPING (GROUPING_ID) on delete cascade
);

create table AGGREGATES (
    AGGREGATES_ID int generated always as identity,
    XXX_ID INT,
    YYY_ID INT,
    NAME VARCHAR(50)
);



The key parts is that AGGREGATES contains data that is grouped by 
YYY_ID (from MAIN) and XXX_ID (from XXX/GROUPING).
So when a row from MAIN is deleted its CORRESPONDING row in AGGREGATE 
needs updating, which I'm doing by doing a delete followed by an INSERT.
The row to delete is the one defined by the corresponding YYY_ID and 
XXX_ID values. And to get the corresponding XXX_ID values I need to 
join back to the GROUPING table. But in the case of a cascade delete 
caused by deletion of a row in GROUPING the row I need to join on is 
no longer present, as its just been deleted.

And I can't do a cascade delete from GROUPING to AGGREGATES as that 
would delete all AGGREGATES with the corresponding XXX_ID, not just 
those with the correct XXX_IDs.

I hope this is clearer!

Tim





Hi Tim,

I wonder if you could use a BEFORE trigger which buffers some result in 
memory for later use by your AFTER trigger. Something like this:

create procedure PRECOMPUTE( grouping_id int )
language java parameter style java reads sql data
external name 'zz.precompute';

-- precompute the result
CREATE TRIGGER trg_beforedelete1 NO CASCADE BEFORE DELETE ON MAIN
REFERENCING OLD ROW AS OLD  FOR EACH ROW MODE DB2SQL
CALL precompute( OLD.GROUPING_ID );

The deleted rows could even be buffered up in memory by a BEFORE trigger 
and then they could be wrapped by a table function for use in the join 
performed by the AFTER trigger.

Disclaimer: I haven't tried this experiment. I'm just waving my hands.

Hope this helps,
-Rick