From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@gmail.com>
Cc: Vik Fearing <vik.fearing@2ndquadrant.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Code comment change
Date: Mon, 01 Jul 2019 22:28:43 -0400
Message-ID: <2931.1562034523@sss.pgh.pa.us> (raw)
In-Reply-To: <CAH2-WzmA2H+rL-xxF5o6QhMD+9x6cJTnz2Mr3Li_pbPBmqoTBQ@mail.gmail.com>
References: <150d3e9f-c7ec-3fb3-4fdb-def47c4144af@2ndquadrant.com>
<CA+hUKGLSbKkSeGmkMAV=woD8wNKnMq3rYqoBeQwvysgDEgV8Lg@mail.gmail.com>
<CAH2-WzmA2H+rL-xxF5o6QhMD+9x6cJTnz2Mr3Li_pbPBmqoTBQ@mail.gmail.com>
Peter Geoghegan <pg@bowt.ie> writes:
> On Sun, Jun 23, 2019 at 3:36 AM Thomas Munro <thomas.munro@gmail.com> wrote:
>> Pushed. Thanks!
> I wonder what the comment is supposed to mean.
> I think that it's addressing the situation prior to commit 70508ba7aed
> in 2003, which was the point when the "fast" root concept was
> introduced.
Yeah. I did some research into the provenance of that comment when
Thomas pushed the change. It's *old*. The whole para exists verbatim
in Postgres v4r2, src/backend/access/nbtree/nbtpage.c dated 1993-12-10
(in my copy of that tarball). The only change since then has been to
change the whitespace for 4-space tabs.
Even more interesting, the same para also exists verbatim in
v4r2's src/backend/access/nobtree/nobtpage.c, which is dated 1991-10-29
in the same tarball. (If you're wondering, "nobtree" seems to stand
for "no-overwrite btree"; so I suppose it went the way of all flesh
when Stonebraker lost interest in write-once mass storage.) So presumably
this comment dates back to some common ancestor of the mainline btree code
and the no-overwrite code, which must have been even older than the 1991
date.
This is only marginally relevant to what we should do about it today,
but I think it's reasonable to conclude that the current locking
considerations are nearly unrelated to what they were when the comment
was written.
> I think that the whole sentence about "the standard class of race
> conditions" should go. There is no more dance. Nothing in
> _bt_getroot() is surprising to me. The other comments explain things
> comprehensively.
+1
regards, tom lane
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Cc: tgl@sss.pgh.pa.us, pg@bowt.ie, thomas.munro@gmail.com, vik.fearing@2ndquadrant.com, pgsql-hackers@lists.postgresql.org
Subject: Re: Code comment change
In-Reply-To: <2931.1562034523@sss.pgh.pa.us>
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