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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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Derby currently does not support creating a unique constraint on a column with
nullable keys, and sql defines what the behavior should be in this case which I
believe is the intent of
this work. So in the case of constraints I don't think additional syntax is
necessary, internally
we would use this new implementation to allow a unique constraint that
supported duplicate
constraints.
If we decide to also to allow one to create indexes through the create index
command that
match this behavior then I agree additional syntax should be used.
I also agree that this implementation would lead to a "third" kind of index and
that should be somehow reflected in the system catalog description of the
index. Using this the optimizer
should be able to make correct assumptions about non-null uniqueness. Some
work will
have to be done to code that tries to use existing indexes if a user asks for
the same kind of index to be created to understand the difference between this
index and others.
I didn't put much thought to the create index behavior. The storage system
does provide a way to optimize building indexes, it allows the tree to one
loaded at create time by providing
a sorted stream of rows. The btree itself does not own how the stream is
produced. It already does a sanity check on each adjacent row to insure the
sort of the keys, so it would be simple to add a different check in the case of
this new btree type. See
BTreeController.java!do_load_insert().
a Derby does sometimes optimize this by throwing all rows through the sorter
and then builds the tree from the bottom up. As was suggested this method
could still be used at the cost of adding a single compare of
each row to it's previous if it has no null in it. The system already does a
compare at this
stage to insure
> 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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