Should I use a unique constraint in a table even though it isn't necessarily required? - sql

Question

In Microsoft SQL Server, when creating tables, are there any downsides to using a unique constraint on a column even though you don't really need it to be unique?
An example would be descriptions for say a role in a user management system:
CREATE TABLE Role
(
ID TINYINT PRIMARY KEY NOT NULL IDENTITY(0, 1),
Title CHARACTER VARYING(32) NOT NULL UNIQUE,
Description CHARACTER VARYING(MAX) NOT NULL UNIQUE
)
My fear is that validating this constraint when doing frequent insertions in other tables will be a very time consuming process. I am unsure as to how this constraint is validated, but I feel like it could be done in a very efficient way or as a linear comparison.

Answer 1

Your fear becomes true: UNIQUE constraint are implemented as indices, and this is time and space consuming.
So, whenever you insert a new row, the database have to update the table, and also one index for each unique constraint.
So, according to you:
using a unique constraint on a column even though you don't really need it to be unique
the answer is no, don't use it. there are time and space downsides.
Your sample table would need a clustered index for the Id, and 2 extra indices, one for each unique constraint. This takes up space, and time to update the 3 indices on the inserts.
This would only be justified if you made queries filtering by those fields.
BY THE WAY:
The original post sample table have several flaws:
that syntax is not SQL Server syntax (and you tagged this as SQL Server)
you cannot create an index in a varchar(max) column
If you correct the syntax and create this table:
CREATE TABLE Role
(
ID tinyint PRIMARY KEY NOT NULL IDENTITY(0, 1),
Title varchar(32) NOT NULL UNIQUE,
Description varchar(32) NOT NULL UNIQUE
)
You can then execute sp_help Role and you'll find the 3 indices.

Answer 2

The database creates an index which backs up the UNIQUE constraint, so it should be very low-cost to do the uniqueness check.
http://msdn.microsoft.com/en-us/library/ms177420.aspx
The Database Engine automatically creates a UNIQUE index to enforce the uniqueness requirement of the UNIQUE constraint. Therefore, if an attempt to insert a duplicate row is made, the Database Engine returns an error message that states the UNIQUE constraint has been violated and does not add the row to the table. Unless a clustered index is explicitly specified, a unique, nonclustered index is created by default to enforce the UNIQUE constraint.

Answer 3

Is it typically a good practice to constrain it if you know the data
will always be unique but it doesn't necessarily need to be unique for
the application to function correctly?
My question to you: would it make sense for two roles to have different titles but the same description? e.g.
INSERT INTO Role ( Title , Description )
VALUES ( 'CEO' , 'Senior manager' ),
( 'CTO' , 'Senior manager' );
To me it would seem to devalue the use of the description; if there were many duplications then it might make more sense to do something more like this:
INSERT INTO Role ( Title )
VALUES ( 'CEO' ),
( 'CTO' );
INSERT INTO SeniorManagers ( Title )
VALUES ( 'CEO' ),
( 'CTO' );
But then again you are not expecting duplicates.
I assume this is a low activity table. You say you fear validating this constraint when doing frequent insertions in other tables. Well, that will not happen (unless there is a trigger we cannot see that might update this table when another table is updated).
Personally, I would ask the designer (business analyst, whatever) to justify not applying a unique constraint. If they cannot then I would impose the unqiue constraint based on common sense. As is usual for such a text column, I would also apply CHECK constraints e.g. to disallow leading/trailing/double spaces, zero-length string, etc.

Answer 4

On SQL Server, the data type tinyint only gives you 256 distinct values. No matter what you do outside of the id column, you're not going to end up with a very big table. It will surely perform quickly even with a dozen indexed columns.
You usually need at least one unique constraint besides the surrogate key, though. If you don't have one, you're liable to end up with data like this.
1 First title First description
2 First title First description
3 First title First description
...
17 Third title Third description
18 First title First description
Tables that permit data like that are usually wrong. Any table that uses foreign key references to this table won't be able to report correctly, say, the number of "First title" used.
I'd argue that allowing multiple, identical titles for roles in a user management system is a design error. I'd probably argue that "title" is a really bad name for that column, too.