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

From: Peter Geoghegan <pg(at)bowt(dot)ie>
To: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Cc: Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Claudio Freire <klaussfreire(at)gmail(dot)com>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>, "Andrey V(dot) Lepikhov" <a(dot)lepikhov(at)postgrespro(dot)ru>
Subject: Re: Making all nbtree entries unique by having heap TIDs participate in comparisons
Date: 2018-07-02 17:43:30
Message-ID: CAH2-Wzm6D=KnV+P8bZE-ZtP4e+W64HtVTdOenqd1d7HjJL3xZQ@mail.gmail.com
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On Thu, Jun 14, 2018 at 11:44 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> I attach an unfinished prototype of suffix truncation, that also
> sometimes *adds* a new attribute in pivot tuples. It adds an extra
> heap TID from the leaf level when truncating away non-distinguishing
> attributes during a leaf page split, though only when it must. The
> patch also has nbtree treat heap TID as a first class part of the key
> space of the index. Claudio wrote a patch that did something similar,
> though without the suffix truncation part [2] (I haven't studied his
> patch, to be honest). My patch is actually a very indirect spin-off of
> Anastasia's covering index patch, and I want to show what I have in
> mind now, while it's still swapped into my head. I won't do any
> serious work on this project unless and until I see a way to implement
> retail index tuple deletion, which seems like a multi-year project
> that requires the buy-in of multiple senior community members. On its
> own, my patch regresses performance unacceptably in some workloads,
> probably due to interactions with kill_prior_tuple()/LP_DEAD hint
> setting, and interactions with page space management when there are
> many "duplicates" (it can still help performance in some pgbench
> workloads with non-unique indexes, though).

I attach a revised version, which is still very much of prototype
quality, but manages to solve a few of the problems that v1 had.
Andrey Lepikhov (CC'd) asked me to post any improved version I might
have for use with his retail index tuple deletion patch, so I thought
I'd post what I have.

The main development for v2 is that the sort order of the implicit
heap TID attribute is flipped. In v1, it was in "ascending" order. In
v2, comparisons of heap TIDs are inverted to make the attribute order
"descending". This has a number of advantages:

* It's almost consistent with the current behavior when there are
repeated insertions of duplicates. Currently, this tends to result in
page splits of the leftmost leaf page among pages that mostly consist
of the same duplicated value. This means that the destabilizing impact
on DROP SCHEMA ... CASCADE regression test output noted before [1] is
totally eliminated. There is now only a single trivial change to
regression test "expected" files, whereas in v1 dozens of "expected"
files had to be changed, often resulting in less useful reports for
the user.

* The performance regression I observed with various pgbench workloads
seems to have gone away, or is now within the noise range. A patch
like this one requires a lot of validation and testing, so this should
be taken with a grain of salt.

I may have been too quick to give up on my original ambition of
writing a stand-alone patch that can be justified entirely on its own
merits, without being tied to some much more ambitious project like
retail index tuple deletion by VACUUM, or zheap's deletion marking. I
still haven't tried to replace the kludgey handling of unique index
enforcement, even though that would probably have a measurable
additional performance benefit. I think that this patch could become
an unambiguous win.

[1] https://postgr.es/m/CAH2-Wz=wAKwhv0PqEBFuK2_s8E60kZRMzDdyLi=-MvcM_pHN_w@mail.gmail.com
--
Peter Geoghegan

Attachment Content-Type Size
v2-0001-Ensure-nbtree-leaf-tuple-keys-are-always-unique.patch application/octet-stream 63.2 KB

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