pgsql: Add third idea about pulling data from indexes.

Log Message:
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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.

Modified Files:
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    pgsql/doc/src/FAQ:
        TODO.html (r1.711 -> r1.712)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html?r1=1.711&r2=1.712)