On Fri, Mar 8, 2019 at 10:48 AM Peter Geoghegan <pg@bowt.ie> wrote:
> > Question: Wouldn't it be more straightforward to use "1 +inf" as page
> > 1's high key? I.e treat any missing columns as positive infinity.
>
> That might also work, but it wouldn't be more straightforward on
> balance. This is because:
I thought of another reason:
* The 'Add high key "continuescan" optimization' is effective because
the high key of a leaf page tends to look relatively dissimilar to
other items on the page. The optimization would almost never help if
the high key was derived from the lastleft item at the time of a split
-- that's no more informative than the lastleft item itself.
As things stand with the patch, a high key usually has a value for its
last untruncated attribute that can only appear on the page to the
right, and never the current page. We'd quite like to be able to
conclude that the page to the right can't be interesting there and
then, without needing to visit it. Making new leaf high keys "as close
as possible to items on the right, without actually touching them"
makes the optimization quite likely to work out with the TPC-C
indexes, when we search for orderline items for an order that is
rightmost of a leaf page in the orderlines primary key.
And another reason:
* This makes it likely that any new items that would go between the
original lastleft and firstright items end up on the right page when
they're inserted after the lastleft/firstright split. This is
generally a good thing, because we've optimized WAL-logging for new
pages that go on the right. (You pointed this out to me in Lisbon, in
fact.)
--
Peter Geoghegan
