From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Michael Paquier <michael@paquier.xyz>,
    Thomas Munro <thomas.munro@gmail.com>,
    Robert Haas <robertmhaas@gmail.com>,
    "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Attempt to consolidate reading of XLOG page
In-reply-to: <20191122142528.GA6465@alvherre.pgsql>
References: <20191122142528.GA6465@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
   message dated "Fri, 22 Nov 2019 11:25:28 -0300."
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Date: Fri, 22 Nov 2019 16:55:20 +0100
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Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

> On 2019-Nov-22, Michael Paquier wrote:
> 
> > On Fri, Nov 22, 2019 at 10:35:51AM -0300, Alvaro Herrera wrote:
> > > FWIW I think the new code is buggy because it doesn't seem to be setting
> > > ws_off, so I suppose the optimization in ReadPageInternal to skip
> > > reading the page when it's already the page we have is not hit, except
> > > for the first page in the segment.  I didn't verify this, just my
> > > impression while reading the code.
> > 
> > FWIW, this matches with my impression here, third paragraph:
> > https://www.postgresql.org/message-id/20191120083802.GB47145@paquier.xyz
> 
> Ah, right.

As I pointed out in

https://www.postgresql.org/message-id/88183.1574261429%40antos

seg.ws_off only replaced readOff in XLogReaderState. So we should only update
ws_off where readOff was updated before commit 709d003. This does happen in
ReadPageInternal (see HEAD) and I see no reason for the final patch to update
ws_off anywhere else.

> I was wondering if we shouldn't do away with the concept of "offset" as
> such, since the offset there is always forcibly set to the start of a
> page.  Why don't we count page numbers instead?  It seems like the
> interface is confusingly generic (measure in bytes) yet not offer any
> extra functionality that could not be obtained with a simpler struct
> repr (measure in pages).

Yes, I agree that page numbers would be sufficient.

> But then that's not something that we need to change in this patch.

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com