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https://issues.apache.org/jira/browse/DERBY-789?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12840543#action_12840543
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Knut Anders Hatlen commented on DERBY-789:
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PostgreSQL 8.4 "forgets" the redundant constraint. That is, for the following
statement
CREATE TABLE T (X INT, CONSTRAINT PK PRIMARY KEY (X), CONSTRAINT U UNIQUE (X))
it creates the primary key constraint PK, but not the unique constraint U.
Attempts to drop the constraint U fail, because there is no such constraint.
Dropping the constraint PK is enough to allow you to insert duplicates.
MySQL 5.1 does not forget the redundant constraint. So there you are not able
to insert duplicates unless you drop *both* the primary key constraint and the
unique constraint.
Derby with the PrimaryImpliesUnique patch behaves the same way as MySQL.
> Usability issue: "Constraints have the same set of columns"
> -------------------------------------------------------------
>
> Key: DERBY-789
> URL: https://issues.apache.org/jira/browse/DERBY-789
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Reporter: Øystein Grøvlen
> Assignee: Bryan Pendleton
> Priority: Minor
> Attachments: PrimaryImpliesUnique.diff
>
>
> Legolas Woodland reported on derby-user that derby return errors like :
> org.apache.derby.client.am.SqlException: Constraints
> 'SQL060103004635123' and 'SQL060103004635121' have the same set of
> columns, which is not allowed.
> He got this when creating a table like this:
> create table WEBSITES (USERID integer not null unique, WEBSITEID
> bigint not null unique, DOMAINNAME varchar(255) not null unique,
> DESCRIPTION varchar(255), PPVIEW double, PPCLICK double, PPWEEK
> double, totalClick bigint, totalView bigint, active smallint, primary
> key (WEBSITEID));
> Omitting the unique specifier made things work.
> I think this is a usability issue. At least, one should not present names
> to the user, that has been generated internally. Instead, it would be
> helpful if the names of the columns involved was mentioned. I see two ways to
> solve this:
> 1. Return error that says that duplicate contraints on the following columns
> are not allowed.
> 2. Allow this and use same index for both constraints. (I guess dropping
> constraints will be more complicated in this case since one will have to
> check if other constraints are using the same index.)
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