I have a big table with 250 columns. We want to add many more columns to this table. But instead of doing this, a new table will be added just a extension of the existing one.
What should be the PK on that extension table same as the main table ? Should that be called FK so that it will be populated with main table then should I have additional primary key on extension table ?

main table ( X Primary key)
x y
1 2
2 2
3 4

extension table ( X will be common between the two tables with same corresponding values, unique and not null)
x z
1 9
2 7
3 5

What are the advantages/ disadvantages of doing this ?
What is the limit of number of columns in Oracle 8.0.6 tables ?

Thanks
Sonali