Add third idea about pulling data from indexes.

Add third idea about pulling data from indexes.

>   A third idea would be for a heap scan to check if all rows are visible
>   and if so set a per-table flag which can be checked by index scans.
>   Any change to the table would have to clear the flag.  To detect
>   changes during the heap scan a counter could be set at the start and
>   checked at the end --- if it is the same, the table has not been
>   modified --- any table change would increment the counter.