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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12539050
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Øystein Grøvlen commented on DERBY-2212:
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Thanks for the new version of the func spec, Anurag.
In this version you do not mention anything about 'CREATE UNIQUE INDEX'. I
guess that means that you propose to keep the existing behavior, and not
introduce a new syntax to match the behavior of unique constraints for nullable
columns. I think it would be good if that was stated explicitly in the func
spec, and you should also explain why you think a new CREATE INDEX is not
necessary. (And if this is no longer about CREATE INDEX, maybe this func spec
is attached to the wrong issue?)
Under 'Hard Upgrade' you write: 'No update will be required as this won't
change the existing behavior'. I am not quite sure what that is supposed to
mean. Do you mean that a user will not have to make any changes to her
applications after a hard upgrade? And what do you mean by 'existing
behavior'? That constraints defined as 'UNIQUE NOT NULL' will work as before?
I think it need to be defined what is meant by "existing behavior".
Another issue that is not clear to me, is the implications of changing the
nullability of a column with ALTER TABLE . It should probably be discussed how
that will be affected by this work.
> Add "Unique where not null" to create index
> -------------------------------------------
>
> Key: DERBY-2212
> URL: https://issues.apache.org/jira/browse/DERBY-2212
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 10.2.1.6
> Reporter: Oleksandr Alesinskyy
> Assignee: Anurag Shekhar
> Attachments: derby-2212preview.diff, derby-2212preview2.diff,
> FunctionalSpec.html, FunctionlaSpecv2.html
>
>
> Derby prohibits creation of unique constraints on nullable colums (as well if
> only some columns in the constraint list are nullable) and treat nulls in
> unique indexes as normal values (i.e. only one row with null values in
> indexed columns may be inserted into the table). This bahavior is very
> restrictive, does not completely comply with SQL standards (both letter and
> intent) as well as with business needs and intending meaning of NULL values
> (2 null values are not considered as equal, this comparision shall return
> NULL, and for selection criteria boolean null is treated as FALSE).
> This behavior, as far as I can see, is modelled after DB2 (and differs from
> behavior of most other major databases, like SyBase, Oracle, etc.).
> But even DB2 provide some means to alleviate these restrictions, namely
> "UNIQUE WHERE NOT NULL" clause for CREATE INDEX statement.
> It will be very good if such "UNIQUE WHERE NOT NULL" clause will be
> introduced in Derby.
> Regards,
> Oleksandr Alesinskyy
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