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https://issues.apache.org/jira/browse/DERBY-3330?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12560070#action_12560070
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Anurag Shekhar commented on DERBY-3330:
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I am following Mike's approach ie internally unique constraint will
be backed by no unique index but during insert, checking the
immediate left and right slot to find duplicate (if there is no null
in the new key). If a duplicate key is found it the insert will be rejected.
I will be introducing a new attribute in BTree to tell the insert routine
that its not a non unique index but almost unique index. New
attribute in BTree will be a persistant attribute so the file system will
not be compatible with the older versions and will require a hard upgrade
routine to migrate the old indexes to new version.
While creating the constraint on a table with existing records
merge short is perform to sort the keys before creating index. A
new merge short class will be required to sort this almost unique
index (allowing duplicates only if there is null in it).
> provide support for unique constraint over nullable columns
> -----------------------------------------------------------
>
> Key: DERBY-3330
> URL: https://issues.apache.org/jira/browse/DERBY-3330
> Project: Derby
> Issue Type: New Feature
> Components: Store
> Affects Versions: 10.4.0.0
> Environment: all
> Reporter: Anurag Shekhar
> Assignee: Anurag Shekhar
>
> Allow unique constraint over nullable field. Right now derby support unique
> constraint only over not null columns.
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