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From "Rick Hillegas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-2212) Add "Unique where not null" to create index
Date Fri, 03 Nov 2017 02:47:00 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16237007#comment-16237007
] 

Rick Hillegas commented on DERBY-2212:
--------------------------------------

Right. The unique index treats NULL as a distinct, known value. In contrast, the unique constraint
treats NULL as an unknown value. Duplicate violations occur when known values collide. The
good news is that a unique constraint creates a backing index which the query optimizer uses
in exactly the same way as an ordinary index. It seems to me that the unique constraint will
give you the duplicate checking you want (nulls do not raise duplicate violations) combined
with the query optimizations you need. Is there some reason that your application will not
behave as you want if you declare a unique constraint rather than a unique index?

> 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
>            Priority: Major
>              Labels: derby_triage10_10
>         Attachments: FunctionalSpec.html, FunctionalSpecV3.html, FunctionalSpecV3_comment.html,
FunctionlaSpecv2.html, 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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