Date: Tue, 9 Mar 2021 11:23:42 -0500
From: Bruce Momjian <bruce@momjian.us>
To: "Bossart, Nathan" <bossartn@amazon.com>
Cc: Andres Freund <andres@anarazel.de>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>,
	"McAlister, Grant" <grant@amazon.com>,
	"Mlodgenski, Jim" <mlodj@amazon.com>,
	"Nasby, Jim" <nasbyj@amazon.com>, "Hsu, John" <hsuchen@amazon.com>
Subject: Re: partial heap only tuples
Message-ID: <20210309162342.GA26575@momjian.us>
References: <2ECBBCA0-4D8D-4841-8872-4A5BBDC063D2@amazon.com>
 <20210213162623.4upmsuzwv2anqoqx@alap3.anarazel.de>
 <962E672B-E874-4EF3-AEE0-85E54AA083A9@amazon.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <962E672B-E874-4EF3-AEE0-85E54AA083A9@amazon.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Archived-At: 
 <https://www.postgresql.org/message-id/20210309162342.GA26575%40momjian.us>
Precedence: bulk

On Mon, Feb 15, 2021 at 08:19:40PM +0000, Bossart, Nathan wrote:
> Yeah, this is something I'm concerned about.  I think adding a bitmap
> of modified columns to the header of PHOT-updated tuples improves
> matters quite a bit, even for single-page vacuuming.  Following is a
> strategy I've been developing (there may still be some gaps).  Here's
> a basic PHOT chain where all tuples are visible and the last one has
> not been deleted or updated:
> 
> idx1    0       1               2       3
> idx2    0       1       2
> idx3    0               2               3
> lp      1       2       3       4       5
> tuple   (0,0,0) (0,1,1) (2,2,1) (2,2,2) (3,2,3)
> bitmap          -xx     xx-     --x     x-x

First, I want to continue encouraging you to work on this because I
think it can yield big improvements.  Second, I like the wiki you
created.  Third, the diagram above seems to be more meaningful if read
from the bottom-up.  I suggest you reorder it on the wiki so it can be
read top-down, maybe:

> lp      1       2       3       4       5
> tuple   (0,0,0) (0,1,1) (2,2,1) (2,2,2) (3,2,3)
> bitmap          -xx     xx-     --x     x-x
> idx1    0       1               2       3
> idx2    0       1       2
> idx3    0               2               3

Fourth, I know in the wiki you said create/drop index needs more
research, but I suggest you avoid any design that will be overly complex
for create/drop index.  For example, a per-row bitmap that is based on
what indexes exist at time of row creation might cause unacceptable
problems in handling create/drop index.  Would you number indexes?  I am
not saying you have to solve all the problems now, but you have to keep
your eye on obstacles that might block your progress later.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee