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.
>

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