[
https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12536960
]
Øystein Grøvlen commented on DERBY-2212:
----------------------------------------
I agree that we should try to avoid incompatibilities with older releases, but
I am just as worried that a slightly different semantics for unique constraints
and unique index will be confusing to users regardless of whether we add
another syntax or not.
Also, Derby will today not allow the explicit creation of an index if such an
index already has been implicitly created through the definition of a
constraint. How should this behave if we keep the old index behavior? Should
we allow two different kinds of unique indexes on the same columns?
With respect to upgrade, I assume that after soft upgrade only the old type of
unique indexes should be created.
After hard upgrade, you should get the new behavior. The question is whether
we should replace any of the old unique indexes with the new type on hard
upgrade. I am not sure I see any good reasons for doing that. Can not we
leave it to the user to decide whether he/she wants to drop and recreated them?
> 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
>
>
> 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
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
