> I have a stituation here. I am just posting this message
> because I want to understand and share with all other the
> performance issue regarding tables and indexes.
>
> I am developing an application where performance is the most
> important issue.
>
> I have one table called CO = "customer order"
> I have one table called CO_ITEM = "the items of the order CO"
>
> I have one table called INV = "invoice"
> I have one table called INV_ITEM = "the items of the invoice INV"
>
> Now happens that the difference between CO and INV is just
> some fields.
> Most fields are common for both tables.

I would like it as it is, as there is little performance to be gained from
combining the two. With something like DB2 you could use object-relational
features to create types with the common fields and subtypes with the
additional fields, but with Firebird this is not possible, nor is it really
necessary. SQL tables are sets. Putting customer orders and invoices
together in one table means you have a mixed set and a broken design with no
real performance benefit. If you want performance, make sure your server
has fast disks, lots of RAM and a fast processor, in that order.