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From "Anurag Shekhar (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2212) Add "Unique where not null" to create index
Date Wed, 17 Oct 2007 07:30:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12535435
] 

Anurag Shekhar commented on DERBY-2212:
---------------------------------------

thanks Mike for revisiting the issue.

I felt letting the index remain unique will be better because 

1. It avoids the need of rescanning the index to check for duplicates after every update/insert.
2. Marking the index as non unique will cause optimizer to conclude that there might be 
duplicate records for equal predicates (not just IS NULL case) and hence it may some 
times select a genuine non unique index when it could have selected this (almost unique)
 index. Right now (in this patch) only if there is a IS NULL in where clause optimizer considers

it  to have multiple records.

Many internal routine don't use optimizer to look for records but directly calls out to internal
store 
methods (like the  case of drop table). In such cases changes will be required irrespective
of 
the approach we use.

> 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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