Update the Create Index statement in the Derby documentation with additional
information
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Key: DERBY-539
URL: http://issues.apache.org/jira/browse/DERBY-539
Project: Derby
Type: Improvement
Components: Documentation
Reporter: Susan Cline
Priority: Minor
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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