Re: Questions about horizontal partitioning

> However,  if the primary key is entirely within those six columns,  there
> will have to be an index on it in both tables to enforce the primary key
> constraint.  In that case,  an inner join could be performed with an index
> lookup or an index scan plus hash join,  for a query that didn't use any
> other columns.  Whether that translates into a significant I/O reduction
> depends on how wide and how frequently non-NULL those other columns are.
>
>

... if someone is feeling pedagogical (and the answer isn't that
complicated), could they explain why a simple index on the desired
columns wouldn't be the best solution?
Cheers
Antoine