On Mon, Mar 14, 2011 at 4:18 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> I'm not quite sure how we'd decide whether to do a "really lazy"
>> vacuum or the kind we do now. The case where this approach wins big
>> is when there are few or no dead tuples. In that case, we do a lot of
>> work looking at the indexes and we don't get much out of it; plus we
>> scan the heap twice instead of just once.
>
> Um, if there are *no* dead tuples then we don't look at the indexes
> anyway ...
But you do still have to scan the heap twice.
> except for the vacuum-cleanup pass which I don't think you get
> to decide you don't need. (You certainly don't get to decide that
> unilaterally without the index AM's cooperation.) I'm less than
> convinced that there's much gold to be mined here.
I'm not sure about that either, although I'm not sure of the reverse
either. But before I invest any time in it, do you have any other
good ideas for addressing the "it stinks to scan the entire index
every time we vacuum" problem? Or for generally making vacuum
cheaper?
--
Robert Haas
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