[ http://issues.apache.org/jira/browse/DERBY-539?page=all ]
Jeff Levitt updated DERBY-539:
------------------------------
Attachment: derby539.diff
rrefsqlj20937.html
Attached patch adds the sentences and modifies the existing text as requested.
Please review the included HTML files to approve the commit. Thanks!
> Update the Create Index statement in the Derby documentation with additional
> information
> ----------------------------------------------------------------------------------------
>
> Key: DERBY-539
> URL: http://issues.apache.org/jira/browse/DERBY-539
> Project: Derby
> Type: Improvement
> Components: Documentation
> Reporter: Susan Cline
> Priority: Minor
> Attachments: derby539.diff, rrefsqlj20937.html
>
> In the 'Create Index' statement documentation of the 10.1 Reference Guide
> (derby/docs/10.1/ref/rrefsqlj20937.html)
> this statement is made about creating indexes and constraints:
> Indexes and constraints
> Unique, primary key, and foreign key constraints generate indexes that
> enforce or "back" the constraint (and are thus sometimes called backing
> indexes). If a column or set of columns has a UNIQUE or PRIMARY KEY
> constraint on it, you can not create an index on those columns. Derby has
> already created it for you with a system-generated name.
> This is true, but I think it can be expanded upon to be clearer. A
> suggestion for this is below:
> Indexes and constraints
> Unique, primary key, and foreign key constraints generate indexes that
> enforce or "back" the constraint (and are thus sometimes called backing
> indexes).
> If a column or set of columns has a PRIMARY KEY constraint on it, you can not
> create an index on those columns. If a column or set of columns has a UNIQUE
> constraint on it, you can not create an index on those columns, but you can
> create
> a PRIMARY KEY constraint on it. Addtionally, if this is the case, a backing
> index
> will be created for the PRIMARY KEY constraint so two indexes will now exist
> on the column or set of columns that had the UNIQUE constraint on it.
> This issue came up when I noticed that I could create a unique index on a
> column, then create a PK on that column. When I used a tool to generate DDL
> for the table I noticed one constraint and two indexes on the column which
> didn't make sense at first when reading the existing documentation. With the
> additional information above I think it explains the real behaviour better.
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