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https://issues.apache.org/jira/browse/DERBY-4988?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12989502#comment-12989502
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Dag H. Wanvik commented on DERBY-4988:
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Thanks for the patch, Mamta! I am going to read through it as as soon as I find
the time.
It's good that you have added so many comments throughout, that is a great
help. Sometimes, I find them a bit dense to read, though: perhaps it would
improve legibility to always add a space between the comment start "//" and the
text. And throw in some blank lines where natural, too, maybe. :)
> ALTER TABLE DROP COLUMN should make use of information in SYSTRIGGERS to
> detect column used through REFERENCING clause to find trigger dependencies
> ---------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: DERBY-4988
> URL: https://issues.apache.org/jira/browse/DERBY-4988
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.7.1.1
> Reporter: Mamta A. Satoor
> Assignee: Mamta A. Satoor
>
> At the time of ALTER TABLE DROP COLUMN, Derby checks if the column being
> dropped in a trigger column and if so, then it will not drop the column if it
> is being done in RESTRICT mode or it will drop the trigger while dropping the
> column in CASCADE mode. This does not implement SQL standard to it's
> entirety.
> **************************************
> SQL standard says following about ALTER TABLE DROP COLUMN RESTRICT and
> trigger dependency in CREATE TRIGGER section
> If RESTRICT is specified, then C shall not be referenced in any of the
> following
> d) Either an explicit trigger column list or a triggered action column set of
> any trigger descriptor.
> (The triggered action column set included in the trigger descriptor is the
> set of all distinct, fully qualified names of columns contained in the
> <triggered action>.)
> **************************************
> What is missing from Derby implementation from SQL standard point of view is
> detected triggered action column set.
> Starting 10.7(with DERBY-1482), Derby started keeping track of trigger action
> columns which are referenced through REFERENCING clause. This information can
> be used to improve the behavior of ALTER TABLE DROP COLUMN in 10.7 and
> higher. This will not cover all the trigger action columns since columns
> referenced without the REFERENCING clause are not tracked anywhere at this
> point. More work will need to be done to implement SQL standard completely.
> But we can take a step forward by using the information available in 10.7 and
> higher to detect trigger action columns which are referenced through
> REFERENCING clause
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