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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-2212) Add "Unique where not null" to create index
Date Tue, 16 Oct 2007 23:53:50 GMT

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

Mike Matrigali updated DERBY-2212:
----------------------------------


i am not sure any of the following are problems, but things to think about.  These all stem
from now you have a unique index that may return multiple rows for a given value if that value
is null
(or in the more interesting case if one of the columns of a multiple key column is null).
 The current derby system may assume this is not possible and may affect the following areas:

1) optimizer probably will assume 1 row from unique index in this case, now it may have to
use some other estimate when dealing with nulls.  If the index was non-unique rather than
unique
current code would just work.
2) unique index locking isolation level algorithm is different from duplicate index locking
isolation level for serializable.  It may have to now change.  Previously a previous key lock
is not necessary to protect a "phantom" insert range for a unique index - after your change
maybe it is?  Again if the index was marked non-unique at implementation level then current
code would
just work.
3) some language and store positioning code may assume that an exact key on a unique index
will only resolve to one row - now it may not.  i don't know where all this code is just warning
that it is something to look for.  Again if code was marked non-unique vs unique
then code continues to work.

I still think at the store level putting multiple null's in makes the index non-unique and
the code is cleaner if we just treat it that way - rather than special casing the compares
like 
your 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-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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