> I would simply add ATTACHMENT_ID system column (probably with
> appropriate index) to tables declared as temporary and taught

optimizer

> to filter out rows which should not be visible by current

attachment.

Why we need extra info when we already has it ? Each record
version alredy has transaction number. We can track sbm of all
attachmnent transactions to distinguish records that our
attachtment\transaction can see. We can merge this sbm's to global
database-level sbm after attachment end to garbage collect unneeded
records. Only minus i see - is wrong index selectivity

> It looks like such implementation may be done in a few days and

doesn't

> require big modifications of the engine.

Exactly. I think my offer even easier ;)

But i prefer temporary page space for temp data. It's much
efficient especially at cleanup time - we just drop temp table
instance data and index pages - no headache with garbage collection
at all.

Don't forget about readonly database. If they could work with
temporary data (which live not in db file) it can be beautiful, imho

Regards,
Vlad