Re: Basic Q on superfluous primary keys

In response to "Kynn Jones" <kynnjo(at)gmail(dot)com>:

> Consider these two very similar schemas:
>
> Schema 1:
>
>
> CREATE TABLE foo (
> id serial PRIMARY KEY,
> frobnitz character(varying 100) NOT NULL UNIQUE
> );
>
>
> CREATE TABLE bar (
> id serial PRIMARY KEY,
> foo_id int REFERENCES foo(id)
> )
>
>
> Schema 2:
>
>
> CREATE TABLE foo (
> frobnitz character(varying 100) PRIMARY KEY
> );
>
>
> CREATE TABLE bar (
> id serial PRIMARY KEY,
> frobnitz character(varying 100) REFERENCES foo(frobnitz)
> )
>
>
>
>
> The two situations are semantically identical: each record in table bar
> refers to a record in table foo. The difference is that in the first
> schema, this referencing is done through an "artificial" serial-integer
> primary key, while in the second schema this reference is done through a
> data field that happens to be unique and not null, so it can serve as
> primary key.

The first case is call a "surrogate key". A little googling on that term
will turn up a wealth of discussion -- both for and against.

> I find Schema 1 awkward and unnatural; more specifically, foo.id seems
> unnecessary in light of the non-null uniqueness of foo.frobnitz. But I
> remember once reading that "long" fields like foo.frobnitz did not make good
> primary keys.

I had a discussion about this recently on the Drupal mailing lists, at the
end of which I promised to do some benchmarking to determine whether or
not text keys really do hurt performance of indexes. Unfortunately, I
still haven't followed through on that promise -- maybe I'll get to it
tomorrow.

> Is the field foo.id in Schema 1 superfluous? For example, wouldn't the
> referencing from bar to foo really be done "behind the scenes" through some
> hidden field (oid?) instead of through the frobnitz text field? Which of
> the two schemas would give better perfornance?

--
Bill Moran
Collaborative Fusion Inc.

wmoran(at)collaborativefusion(dot)com
Phone: 412-422-3463x4023

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