View of views, what will speed faster or slower why ?

Question

we can create view of view.But will it work faster or it will be slower.
Can any one tell me the reasons.

Answer 1

So it is (I think) generally accepted best practice
to always write views which work only with tables and
not other views. Of course this can lead to lots of
duplication, if you have many views which are
variations on a theme. That seems a bit excessive to me, but perhaps I'm in the minority here.
I would much, for example, have no problem creating multiple layers of views in order to prevent identical logic from getting spread across lots of different views because it's almost inevitable that when the logic changes, the "variation on a theme" views are going to get out of sync.
If you have some concept of "pipeline orders" for orders that are in various stages of the pipeline, I'd have no problem creating a PIPELINE_ORDERS view and having the CUSTOMER_PIPELINE, SALESMAN_PIPELINE, and WAREHOUSE_PIPELINE views reference that view rather than embedding the "what qualifies as a pipeline order" logic in all the views in order to ensure that everyone is using the same definition. I've seen way too many cases where a simple test got coded in slightly different ways in different views resulting in subtly different output that caused subtly different bugs.
Of course, depending on the logic, you might be able to create an IS_PIPELINE function that could be put in all the views in order to centralize the same logic, but that may cause bigger problems if Oracle decides it now has to call the function a bazillion times for the bazillion closed orders in your table. To which, of course, you might decide to create a function-based index on the IS_PIPELINE function, which may or may not be an option depending on the function signature and whether all the criteria are columns in the same table. And you'll probably need to ensure the optimizer knows the return value for this function is skewed (which may be more or less difficult depending on the Oracle version). But you're pretty quickly dealing with performance issues that are at least as complicated as the performance issues you've created for the optimizer by having nested views.
Justin