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