>A new free table is being added to an application. It will be the child of a previously existing table.
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>The parent table unique id is made up of three fields totaling 20 characters. I'm considering adding an auto incrementing integer to the parent table to serve as link to the child table but am afraid what may happen if somehow the id sequence gets out of control and skips ahead to the maximum integer (2^32 - 1), This is a totally critical table in the application.
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>In the experience of those here, is this a realistic fear? I would appreciate comments.

What do you mean skipping ahead?

I use GUIDs, which is 16 chars. It's less than your 20 character key and can be generated by the local workstation.