pgsql: Update count(*) discussion ideas: < BY col {DESC} LIMIT 1.

Log Message:
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Update count(*) discussion ideas:

< BY col {DESC} LIMIT 1. Completing this item involves making this
> BY col {DESC} LIMIT 1. Completing this item involves doing this
< invalidated if anyone modifies the table.
<
> invalidated if anyone modifies the table. Another idea is to
> get a count directly from a unique index, but for this to be
> faster than a sequential scan it must avoid access to the heap
> to obtain tuple visibility information.
>
> * Allow data to be pulled directly from indexes
>
> Currently indexes do not have enough tuple tuple visibility
> information to allow data to be pulled from the index without
> also accessing the heap. One way to allow this is to set a bit
> to index tuples to indicate if a tuple is currently visible to
> all transactions when the first valid heap lookup happens. This
> bit would have to be cleared when a heap tuple is expired.
>

Modified Files:
--------------
pgsql/doc:
TODO (r1.1434 -> r1.1435)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1434&r2=1.1435)