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From "Oleksandr Alesinskyy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2212) Add "Unique where not null" to create index
Date Fri, 19 Oct 2007 17:21:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12536278

Oleksandr Alesinskyy commented on DERBY-2212:

Keeping backward compatibility in this case shall be of much higher priority then keeping
with SQL standard (BTW, each major database deviates from SQL standard at this point). So,
what would we achieve if CREATE INDEX would not provide additional syntax?

There are 2 possibilities

1. CREATE UNIQUE INDEX retain existing behavior. Then indexes created implicitly by UNIQUE
constraint would differ in behavior from indexes created by CREATE UNIQUE INDEX. Customer
headache is ensured.

2. CREATE UNIQUE INDEX creates indexes with new behavior. Then many existing application will
be broken. Instead of headache customers encounter pain in the ass. Not much better.

> 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:
>            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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