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https://issues.apache.org/jira/browse/DERBY-2212?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mike Matrigali updated DERBY-2212:
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There are 2 discussions here. Let's settle first what SQL level behavior is
being implemented. I believe it is clear that the focus is to allow derby to
create unique
constraints on a set of columns where one or more of the columns is nullable.
This
is the SQL standard, and I don't see any backward compatiblity issues. Many
users have
asked for this and it adds to our standards implementation so seems like a good
feature to add to Derby.
My opinion is that we should not change the existing behavior for unique
indexes. Changing
the behavior presents possible application upgrade and backward compatibility
issues. The
change seems minor, but it has been my experience that no matter how small the
behavior change a number of applications will depend on it. I don't think this
is a SQL standard issue
as create index is an implementation specific ddl and is not covered.
I lean toward not providing extra syntax to create index to create the "new"
multiple null unique indexes as it just seems like another way to do the same
thing you can do with a constraint. But I don't really feel strongly about
this one. If someone wanted to do it I would not -1 it. Historically I think
alter table add constraint came after create index, so originally
one needed the create index command to add more unique indexes after the table
was
created.
I do think the decision on support of old unique index may affect best internal
implementation details.
> 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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