Re: Making all nbtree entries unique by having heap TIDs participate in comparisons

On Tue, Oct 23, 2018 at 11:35 AM Andrey Lepikhov
<a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
> I have same problem with background heap & index cleaner (based on your
> patch). In this case the bottleneck is WAL-record which I need to write
> for each cleaned block and locks which are held during the WAL-record
> writing process.

Part of the problem here is that v6 uses up to 25 candidate split
points, even during regularly calls to _bt_findsplitloc(). That was
based on some synthetic test-cases. I've found that I can get most of
the benefit in index size with far fewer spilt points, though. The
extra work done with an exclusive buffer lock held will be
considerably reduced in v7. I'll probably post that in a couple of
weeks, since I'm in Europe for pgConf.EU. I don't fully understand the
problems here, but even still I know that what you were testing wasn't
very well optimized for write-heavy workloads. It would be especially
bad with pgbench, since there isn't much opportunity to reduce the
size of indexes there.

> Maybe you will do a test without writing any data to disk?

Yeah, I should test that on its own. I'm particularly interested in
TPC-C, because it's a particularly good target for my patch. I can
find a way of only executing the read TPC-C queries, to see where they
are on their own. TPC-C is particularly write-heavy, especially
compared to the much more recent though less influential TPC-E
benchmark.

--
Peter Geoghegan