pgsql: Add all heap page rows visible bitmap idea: < the heap.

Log Message:
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Add all heap page rows visible bitmap idea:

< the heap. One way to allow this is to set a bit to index tuples

the heap. One way to allow this is to set a bit on index tuples

< be cleared when a heap tuple is expired.
<

be cleared when a heap tuple is expired. Another idea is to maintain
a bitmap of heap pages where all rows are visible to all backends,
and allow index lookups to reference that bitmap to avoid heap
lookups, perhaps the same bitmap we might add someday to determine
which heap pages need vacuuming.

Modified Files:
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pgsql/doc:
TODO (r1.1708 -> r1.1709)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1708&amp;r2=1.1709)
pgsql/doc/src/FAQ:
TODO.html (r1.215 -> r1.216)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.215&amp;r2=1.216)