Re: does a table always need a PK?

Paul G. Brown wrote:

> What you heard from proponents of sets over bags are all of the good
> things sets give you, but none of the bad things, chiefly the need to
> ensure that each edge in the physical query plan consists of unique
> rows. This means a) figuring out when the edge's set properties are
> guaranteed by the schema semantics (keys, unique columns) or the
> physical database (columns in a unique index appear in the tuple), and
> then when there is no guarantee b) imposing a blocking physical operation
> to guarantee the uniqueness.
>
> Set algebras were tried. They slowed the systems down. Whether or not this
> makes them impractical is an open question but to be honest about it, it
> isn't clear that the much touted advantages of set algebras outweigh their
> disadvantages. Theory is inherently practical.
>

Thanks for interesting posting, Paul! Could you give me more details on that last paragraph? When were
they (set algebras) tried? By the original System-R team? At some later time? Could it be that
it was found hard in the mid 70's but could not be tried again because of SQL-dominance?

regards,
Lauri Pietarinen Received on Sun Aug 31 2003 - 22:53:33 CEST