Avoid scanning on tape

Hi.

I would like to store the "address on tape" of the first and the last tuple for the run I'm currently creating on a
specifictape. 
I need it in order to access directly to those two tuples right after terminating the creation of that specific run.

Of course I could store that address of first tuple as soon as I start the creation of the corresponding new run. On
theother hand I could store the address of that last tuple when I terminate the creation of that run. 

The problem is that Postgres writes tuples in various buffers before really writing blocks of tuples on disk.

Is there an efficient way to access to those tuples avoiding scanning a specific run from the last tuple till the first
one?

Thanks for your attention.

Regards, Manolo.
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