public inbox for [email protected]
help / color / mirror / Atom feedFrom: Tom Lane <[email protected]>
To: Peter Geoghegan <[email protected]>
Cc: Neto pr <[email protected]>
Cc: postgres performance list <[email protected]>
Subject: Re: Pageinspect bt_metap help
Date: Mon, 18 Sep 2017 23:28:19 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAH2-WzmvNvC4FGmixpMAFcq3GiPxMiZqw9-_4Gn_J34rAxg1NQ@mail.gmail.com>
References: <CA+wPC0MRuc0qn6v57VNtBE0Dh-r1Bcugi4FHHqC_kY-1X2tzGQ@mail.gmail.com>
<CAH2-Wzka8Xkb79T5TbTeHPch2dTpWY+HsViqNLtKSKXY9bzOFA@mail.gmail.com>
<CA+wPC0NbHJ=awqjkq-GzosQ1r=nGN=2=zk6x=7btVCk7=4SABg@mail.gmail.com>
<CAH2-WzmvNvC4FGmixpMAFcq3GiPxMiZqw9-_4Gn_J34rAxg1NQ@mail.gmail.com>
List-Unsubscribe: <mailto:[email protected]?body=unsub%20pgsql-performance>
Peter Geoghegan <[email protected]> writes:
> On Mon, Sep 18, 2017 at 7:31 AM, Neto pr <[email protected]> wrote:
>> In my example, the values of fast_root, fast_root are equal to root, level,
>> I believe that due to the newly created index and no delete operations
>> occurred in the table.
> Fast root and true root will probably never be different, even when
> there are many deletions, including page deletions by VACUUM. As I
> understand it, the fast root thing is for a fairly rare, though still
> important edge case. It's a way of working around the fact that a
> B-Tree can never become shorter due to the locking protocols not
> allowing it. We can instead just pretend that it's shorter, knowing
> that upper levels don't contain useful information.
My (vague) recollection is that it's actually useful in cases where the
live key-space constantly migrates to the right, so that the original
upper-level key splits would become impossibly unbalanced. This isn't
all that unusual a situation; consider timestamp keys for instance,
in a table where old data gets flushed regularly.
regards, tom lane
--
Sent via pgsql-performance mailing list ([email protected])
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
reply
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: [email protected]
Cc: [email protected], [email protected], [email protected]
Subject: Re: Pageinspect bt_metap help
In-Reply-To: <[email protected]>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox