Re: Page at a time index scan

I've committed a rewritten version of this patch.

Heikki Linnakangas <hlinnaka@iki.fi> writes:
> On Fri, 5 May 2006, Tom Lane wrote:
>> btbulkdelete arrives at a page, it need take no special action unless the
>> page is newly split *and* its right-link points to a lower physical
>> address.  If that's true, then after vacuuming the page, follow its
>> right-link and vacuum that page; repeat until arriving at a page that is
>> either not newly split or is above the current location of the outer loop.
>> Then return to the outer, sequential-scan loop.

> It'd be a bit more efficient to finish the sequential-scan first, and
> memorize the newly-split pages' right-links as they're encountered. Then
> scan those pages as a separate second pass, or earlier if we run out of
> memory reserved for memorizing them.

I didn't do this.  Aside from the extra memory requirement, it's not
apparent to me that it'd make things faster.  The disadvantage is that
it would require more page reads than the other way: if you visit the
split page immediately, and note that its right-link is above the
current outer loop location, then you can skip following the right-link
because you know you'll visit the page later.  If you postpone then you
have to chase every chain until actually reading a page with an old
cycle ID.  I think this extra I/O would likely outweigh any savings from
not interrupting the main scan.

> If btbulkdelete always clears the marker (assuming zero isn't a valid
> value), 16 bits is plenty. Unless a vacuum is aborted, there should never
> be a value older than current value - 1 in the index. We could live with a
> 2-bit counter.

For the moment, the code is only clearing the marker if it's equal to
the current cycle ID.  This is sufficient to recognize
definitely-already-processed pages, but it doesn't prevent false
positives in general.  If we ever need the space we could narrow the
counter, at the cost of having to expend more I/O to keep the values
cleared.

            regards, tom lane