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From Øystein Grøvlen (JIRA) <j...@apache.org>
Subject [jira] Commented: (DERBY-2212) Add "Unique where not null" to create index
Date Thu, 11 Oct 2007 00:25:50 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12533912
] 

Øystein Grøvlen commented on DERBY-2212:
----------------------------------------

Thanks for the patch, Anurag.  Your approach to solving this seems OK to me.  I have a few
questions and comments:

ControlRow: I do not quite understand all implications of your changes here, and some comments
explaining this would be good. Regarding the following code that, if I have understood thing
correctly, is executed if both key and index row is equal for all columns and there are columns
where both have null:

               if (nullPart != -1) {
			if (ascOrDesc[nullPart])  // true - Ascending order
					return 1;
				else
					return -1;            
                }

1. I do not understand the reasoning behind the return values.  I notice that if I swap them,
dropping a table does not fail anymore (I have not checked if that breaks other operations).
2. What are the implications if there are several columns for which both have null?  You only
test ascOrDesc for the latest column.
3. This code will not be executed for partial key matching.  Hence, null values will always
be treated equal in that case.  Is that intentional?  If yes, it would be good if that would
be clear from the specification of the method, and not just be an implicit assumption based
on how partial key matching is currently used.

DataType#compare: Are you sure that nullsOrderedLow does not matter when both values are null?
I am a bit confused,so it would be good if you could add a convincing comment.  And why return
-1 and not 1 if nulls are not equal?  Does this work equally well for both ascending and descending
scans?

FromBaseTable: The comment above your code needs to be updated to reflect your change.

SortController: Typos: 'defaul', 'other wise', 'returns if nulls are ...' should be 'returns
whether ...', 'true if they ...' should be 'true if null values ...'

Nit: There is a lot of whitespace issues that I hope you will correct before a final patch.


> 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 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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