>>When you say, "only a subset of records would benefit"....are you talking roughly 10%....or 25%.....etc. ?
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>Well, at first, only new records would benefit of it. So, the ratio would be 0.00001 or something like that. Then, it will grow on a steady base. It is highly difficult to say at this point but this new set of data could become a large part of the table.
Is there any chance you might back-fill the older rows?
Consider this - suppose you had decided to add the column years ago. Would a high % of the rows have had a value for this column?
I can't say with certainty, but if this column would have had values had you defined it years ago, I'd say just add the column to the table and don't bother with a 1x1 relationship.