About the FSM :Would it be possible to add a flag marking pages where all tuples are visible to all transactions ? (kinda like frozen I think) This could be useful to implement index-only scans, for count(), or to quickly skip rows when OFFSET is used, or to use only the index when the selected columns are all in the index. Of course if the page is flagged as "may contain updated tuples", then it would have to look in the heap. But, for tables that are not randomly updated (for instance tables that are mostly appended to, like forum posts, or logs, or the huge archive table, etc) it could save a lot of heap lookups and IO.
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