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From "Mamta A. Satoor (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4984) ALTER TABLE DROP COLUMN may leave triggers invalid even if they are not using the column getting dropped.
Date Tue, 25 Jan 2011 22:49:45 GMT
ALTER TABLE DROP COLUMN may leave triggers invalid even if they are not using the column getting
dropped.
---------------------------------------------------------------------------------------------------------

                 Key: DERBY-4984
                 URL: https://issues.apache.org/jira/browse/DERBY-4984
             Project: Derby
          Issue Type: Bug
          Components: SQL
    Affects Versions: 10.7.1.1, 10.6.2.1, 10.6.1.0, 10.5.3.0, 10.5.2.0, 10.5.1.1, 10.4.2.0,
10.4.1.3, 10.3.3.0, 10.3.2.1, 10.3.1.4
            Reporter: Mamta A. Satoor


While doing testing for DERBY-4887, I found a case where ALTER TABLE DROP COLUMN will leave
triggers in invalid state even if those triggers are not using the column getting dropped.
eg
CREATE TABLE tab ( 
       element_id INTEGER NOT NULL, 
       altered_id VARCHAR(30) NOT NULL
); 
insert into tab values(1,'aa');
-- Create a trigger against the table 
CREATE TRIGGER mytrig 
 AFTER UPDATE OF altered_id ON tab 
 REFERENCING NEW AS newt OLD AS oldt 
 FOR EACH ROW MODE DB2SQL 
  SELECT newt.altered_id from tab;
--Drop the first column in the table. This will cause the column positions to be recalculated
within the table
alter table tab drop column element_id; 
--mytrig is still looking for column altered_id at position 2 but drop column has changed
it's position within the table to 1
update tab set altered_id='bb';

As shown in the example above, table "TAB" only has 2 columns. The trigger "MYTRIG" uses the
2nd column in it's trigger action through the REFERENCING clause. During trigger action sql
parsing, every column referenced through REFERENCING clause gets transformed into a reference
to the column through it's column position in the trigger table(this change to look for columns
based on their column positions rather than the name went in as revision 397959 with following
commit comments DERBY-1258 Change the generated code for a new/old column in a row trigger
to access columns by position and not name to avoid the case-insensitive name lookup specified
by JDBC.) When in the script above, we drop the column in position 1, the trigger "MYTRIG"
ends up becoming invalid because column being used in the trigger action is no more in column
position 2.

One possible solution is to regenerate the SPSDescriptor associated with the trigger action
for all the triggers defined on the table whose column is getting dropped. We could be little
smarter and only regenerate the SPSDescriptor for the triggers who use the REFERENCING clause.
But we need to do more testing to make sure that triggers without REFERENCING clause do not
get impacted by a drop of column which is not the last column of the table. This optimization
of recognizing the right triggers may not be worth it since performance may not be that big
a criteria for an ALTER TABLE DROP COLUMN which should be a rare operation in a production
system.

An interim solution to this problem is obviously to drop and recreate the triggers


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