Re: RFC: planner statistics in 7.2

> A different approach that's been discussed on pghackers is to make use
> of btree indexes for columns that have such indexes: we could scan the
> indexes to visit all the column values in sorted order. I have rejected
> that approach because (a) it doesn't help for columns without a suitable
> index; (b) our indexes don't distinguish deleted and live tuples,
> which would skew the statistics --- in particular, we couldn't tell a
> frequently-updated single tuple from a commonly repeated value; (c)
> scanning multiple indexes would likely require more total I/O than just
> grabbing sample tuples from the main table --- especially if we have to
> do that anyway to handle columns without indexes.

Remember one idea is for index scans to automatically update the expired
flag in the index bitfields when they check the heap tuple.

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