Alter Table Vs Ins Select for Modifying Table Structure in Teradata

Conclusion

It is strongly recommend implementing Alter Table, at least start considering it. If you're concerned about availability you should bear in mind that this process will probably be scheduled out of business hours anyway.
And when you need to change the [P]PI or you just want the safeness of a copy of the old table you should definitely prefer Merge Into over good ol' Insert Select.

Please review below figure to see the Pros and Cons of using Alter/Insert Selct and Merge into options for modifying any table in Teradata.

The above conclusion is based on Dieter Blog reference link for details is as follows

http://developer.teradata.com/blog/dnoeth/2013/08/why-dont-you-use-alter-table-to-alter-a-table