You wrote:
Your idea is not bad (I'll test it), but the pair INSERT-
UPDATE I think is very fast, the problem will be the writes to the
disk another time (and now the 8MB is possibly will be 22MB or
more)...
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When inserting in the original table you have the overhead of IB checking
the constraint (of the primary key). With the simple table (just one
field, no constraints), it might be faster.
Run all the inserts in one single transaction (as Geoff mentions).
As for the size, IB uses a simple compression (which might help).

Regards,
Aage J.