Re: Reviewing freeze map code

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Reviewing freeze map code
Date: 2016-07-01 19:18:39
Message-ID: CA+TgmoY8PdS9TxH35_OH9003_BYPj3_E14LiN6Kk5Ad8r_wi8A@mail.gmail.com
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On Fri, Jul 1, 2016 at 10:22 AM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> Ah, you're right, I misunderstood.
>
> Attached updated patch incorporating your comments.
> I've changed it so that heap_xlog_lock clears vm flags if page is
> marked all frozen.

I believe that this should be separated into two patches, since there
are two issues here:

1. Locking a tuple doesn't clear the all-frozen bit, but needs to do so.
2. heap_update releases the buffer content lock without logging the
changes it has made.

With respect to #1, there is no need to clear the all-visible bit,
only the all-frozen bit. However, that's a bit tricky given that we
removed PD_ALL_FROZEN. Should we think about putting that back again?
Should we just clear all-visible and call it good enough? The only
cost of that is that vacuum will come along and mark the page
all-visible again instead of skipping it, but that's probably not an
enormous expense in most cases.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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