[PostgreSQL-Hackers] Get the offset of a tuple inside a table

Hi Szymon, Kevin,
The offset is the order of a tuple in a Sequential Scan.

What I want to do is, for a given key return the tuples that Index scan can
find, and return the rest tuples by a seq scan. So, I need to know which
tuples have been returned by maintain a bitmap, and to avoid return the same
tuple twice.

If I can know the offset of a tuple in the order of file scan, then I can
force the seq scan to skip it.

By ctid, can I know how much tuples are in a block, is it an constant?

I think a more general solution would be build a hash table on ctid.
What do you think?

Thanks