Dane Foster <studdugie@gmail.com> writes:
> My example is modeling an order details table and the answer to the
> question of selectivity is it depends. For some of our clients it is highly
> selective because customers generally order a single item at a time. For
> others it's multi-modal because it starts out w/ their customers ordering
> only a single item but over time customer behavior changes and there is
> this mix of single and multi item orders. Additionally my use case for
> PostgreSQL is the VPS use case where each client has their own schema so
> I'd prefer not to have to deal w/ per client index building and
> maintenance. So is there a rule of thumb design wise for variable
> selectivity as I've described?
See
http://www.postgresql.org/docs/9.4/static/indexes.html
particularly sections 11.3 and 11.5.
regards, tom lane