Thread: redundancy in CHECK CONSTRAINTs

I have the following table:

CREATE TABLE gyuktnine (    id               SERIAL,   intsystem      INTEGER NOT NULL REFERENCES yuksystems(id)
CONSTRAINT
 
int_cannot_equal_ext                      CHECK (intsystem != extsystem),   extsystem     INTEGER NOT NULL REFERENCES
yuksystems(id)CONSTRAINT 
 
ext_cannot_equal_int                       CHECK (extsystem != intsystem),    PRIMARY KEY (intsystem, extsystem)
);

the intsystem and extsystem fields both have a check constraint on them 
which preventing any one record from having values in which they are 
equal. There is also a primary key. Is this redundant? Do only one of 
them really need this constraint? Or does it not really matter. I'm 
concerned about using constraints like this and have redundant checks 
built in slowing down my db.

Ferindo

Ferindo Middleton Jr <fmiddleton@verizon.net> writes:
> I have the following table:

> CREATE TABLE gyuktnine (
>      id               SERIAL,
>     intsystem      INTEGER NOT NULL REFERENCES yuksystems(id) CONSTRAINT 
> int_cannot_equal_ext
>                        CHECK (intsystem != extsystem),
>     extsystem     INTEGER NOT NULL REFERENCES yuksystems(id) CONSTRAINT 
> ext_cannot_equal_int
>                         CHECK (extsystem != intsystem), 
>     PRIMARY KEY (intsystem, extsystem)
> );

> Is this redundant?

Yes.  I think it's poor style too: a constraint referencing multiple
columns should be written as a table constraint not a column constraint.
That is, you ought to write

CREATE TABLE gyuktnine (   id            SERIAL,   intsystem     INTEGER NOT NULL REFERENCES yuksystems(id),
extsystem    INTEGER NOT NULL REFERENCES yuksystems(id),   PRIMARY KEY (intsystem, extsystem),   CONSTRAINT
int_cannot_equal_extCHECK (intsystem != extsystem)
 
);

At least in the earlier versions of the SQL standard, it was actually
illegal for a column constraint to reference any other columns.  I'm not
sure if that's still true in the latest spec.  Postgres treats column
constraints and table constraints alike, but other SQL databases are
likely to be pickier.

BTW, is there any actual need for the "id" column here, seeing that
you have a natural primary key?
        regards, tom lane

Thank you for your advice, Tom. I've re-done the table in my db using 
the schema you describe below. The is a need for the id field. Other 
tables in my applications use it to refer to any one intsystem/extsystem 
relationship and be able to provide users with one simple number to use 
to refer to them. Thank you.

Ferindo

Tom Lane wrote:
> Ferindo Middleton Jr <fmiddleton@verizon.net> writes:
>   
>> I have the following table:
>>     
>
>   
>> CREATE TABLE gyuktnine (
>>      id               SERIAL,
>>     intsystem      INTEGER NOT NULL REFERENCES yuksystems(id) CONSTRAINT 
>> int_cannot_equal_ext
>>                        CHECK (intsystem != extsystem),
>>     extsystem     INTEGER NOT NULL REFERENCES yuksystems(id) CONSTRAINT 
>> ext_cannot_equal_int
>>                         CHECK (extsystem != intsystem), 
>>     PRIMARY KEY (intsystem, extsystem)
>> );
>>     
>
>   
>> Is this redundant?
>>     
>
> Yes.  I think it's poor style too: a constraint referencing multiple
> columns should be written as a table constraint not a column constraint.
> That is, you ought to write
>
> CREATE TABLE gyuktnine (
>     id            SERIAL,
>     intsystem     INTEGER NOT NULL REFERENCES yuksystems(id),
>     extsystem     INTEGER NOT NULL REFERENCES yuksystems(id),
>     PRIMARY KEY (intsystem, extsystem),
>     CONSTRAINT int_cannot_equal_ext CHECK (intsystem != extsystem)
> );
>
> At least in the earlier versions of the SQL standard, it was actually
> illegal for a column constraint to reference any other columns.  I'm not
> sure if that's still true in the latest spec.  Postgres treats column
> constraints and table constraints alike, but other SQL databases are
> likely to be pickier.
>
> BTW, is there any actual need for the "id" column here, seeing that
> you have a natural primary key?
>
>             regards, tom lane
>
>