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From "Rick Hillegas (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3940) Dropping a column does not drop triggers which mention that column
Date Fri, 07 Nov 2008 15:20:52 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12645788#action_12645788
] 

Rick Hillegas commented on DERBY-3940:
--------------------------------------

Here's my quick analysis of the problem.

AlterTableConstantAction.dropColumnFromTable() holds the logic to detect if there are any
triggers which depend on the column being dropped. That routine checks whether the dropped
column is in the array in TriggerDescriptor.getReferencedCols() for any trigger. That, in
turn, translates into whether the dropped column turns up in the ReferencedColumns object
for some trigger. That object is persisted in SYS.SYSTRIGGERS.REFERENCEDCOLUMNS. Unfortunately,
the only columns which are recorded in that object are the columns which appear in the UPDATE
OF clause of the trigger. We don't persistently record the columns which appear in the action
clause of the trigger. Here are some possible solutions to this problem:

1) Persistently record the columns which appear in the action clauses of triggers. This could
involve expanding the meaning of the REFERENCEDCOLUMNS column or adding a new column to SYS.SYSTRIGGERS.
In order to handle pre-existing triggers, the upgrade logic would need to drop and recreate
all triggers.

2) Alternatively, AlterTableConstantAction.dropColumnFromTable() could parse the action statements
of all triggers on the table to see if any of them mention the dropped column.


> Dropping a column does not drop triggers which mention that column
> ------------------------------------------------------------------
>
>                 Key: DERBY-3940
>                 URL: https://issues.apache.org/jira/browse/DERBY-3940
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.4.2.1, 10.5.0.0
>            Reporter: Rick Hillegas
>         Attachments: dropColumnWithTrigger.sql, Triggers.java
>
>
> Put an INSERT trigger on a table and mention a column in the trigger. Then drop that
column from the table. If you drop the column with RESTRICT semantics, you don't get an objection.
Both CASCADE and RESTRICT drop the column. However, the trigger remains in both cases. After
that, INSERTs into the table fail because the trigger can't find the dropped column. The workaround
is to manually drop the trigger either before or after dropping the column. I will attach
a test case.

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