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From "Danoja Dias (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6852) Allow identity columns to cycle (as defined in SQL:2003)
Date Wed, 15 Jun 2016 06:46:09 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6852?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15331246#comment-15331246
] 

Danoja Dias commented on DERBY-6852:
------------------------------------

I looked in to the code and then I found in sequenceGenerator.java this feature is already
implemented there. 

if ( overflowed( _currentValue, nextValue ) )
        {
            // Take this generator offline if we've wrapped around but cycling really isn't
allowed.
            if ( !_CAN_CYCLE )
            {
                markExhausted( retval );
                return;
            }

            // Otherwise, cycling is allowed.
            nextValue = _RESTART_VALUE;
        }

I need to find how  _CAN_CYCLE  variable changes. Here  if cycling is allowed, nextValue is
_RESTART_VALUE that is the value we give to start the identity column. 
I think it must be the minimum value to start.


> Allow identity columns to cycle (as defined in SQL:2003)
> --------------------------------------------------------
>
>                 Key: DERBY-6852
>                 URL: https://issues.apache.org/jira/browse/DERBY-6852
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>            Reporter: Peter Hansson
>
> Currently when an IDENTITY column reaches its maximum value it will produce an error.
> For tables that are used as 'transaction logs' or 'event logs' it often makes sense to
let the table automatically start over with the first identity value again when the max is
reached. This would be similar to the CYCLE option on Oracle's SEQUENCE and as defined in
SQL:2003. And Derby is probably used quite often for this purpose, I guess, perhaps even more
than other RDBMSs.
> At the moment every developer have to program their own logic for this.
> I propose to introduce the CYCLE option.
> The idea of CYCLE is based on the assumption that there's been a prior cleanup in the
table rows so that it will be possible to re-use ids that have been used previously. If that
is not the case - and a rollover happens - then a duplicate value error will occur. In this
sense it can be argued that the CYCLE option will trade a _certain_ error for a _potential_
error. Most Derby users would possibly gladly accept such a bargain. In other words: This
option will greatly enhance the usability of IDENTITY columns.
> The current implementation of IDENTITY columns SQL grammar in Derby is a subset of the
SQL:2003 standard which is the first of the SQL standards to define IDENTITY columns. Interestingly
the standard also defines the CYCLE option but this was never implemented in Derby. Also see
[SQL-99 and SQL-2003 features mapped to Derby|https://wiki.apache.org/db-derby/SQLvsDerbyFeatures]
(scroll to T174).
> In other words: The proposal is simply to implement CYCLE as defined in SQL:2003.



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