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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4988) ALTER TABLE DROP COLUMN should make use of information in SYSTRIGGERS to detect column used through REFERENCING clause to find trigger dependencies
Date Wed, 02 Feb 2011 03:17:29 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4988?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12989502#comment-12989502
] 

Dag H. Wanvik commented on DERBY-4988:
--------------------------------------

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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