> > >First we must run the query in serializable mode and replace the 
> > >snapshot with a synthetic one, which defines visibility at the
> > >of the desired transaction
> > >
> > >probably it is a good idea to take a lock on all tables involved to

> > >avoid a vacuum to be started on them when the query is running.
> > Would the xmin exported by that transaction prevent vacuum from 
> > removing any tuples still needed for the flashback snapshot?
> Sure, and that makes the mentioned lock unnecessary.

Problem is, that that transaction sets a historic snapshot at a later
time, so it is not yet running when vacuum looks at "global xmin".
So something else needs to hold up global xmin (see prev post).


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