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
