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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12536178
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Oleksandr Alesinskyy commented on DERBY-2212:
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Hello Mike,
concerning index and unique constraints - I did not completely catch your
point, sorry.
Do you propose to add additional syntax to CONSTRAINT clause only or just go
Oracle
way which treats all unique indexes the same way?
And I completely agree that before implementation some specification concerning
both
SQL syntax and index behavior shall be agreed.
In my opinion, for compatibility reasons DB2 way is better as any index created
without
additional syntax (does not matter in the constraint clause or in the create
index command)
behave exactly as before, so no existing application would be affected. And
index, created
with additional syntax provides new behavior. BTW, it may be considered as new,
third type
of index (that likely require system dictionary expansion).
Regards,
Oleksandr
> 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
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