| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Peter Geoghegan <pg(at)bowt(dot)ie> |
| Cc: | John Naylor <john(dot)naylor(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Overhauling "Routine Vacuuming" docs, particularly its handling of freezing |
| Date: | 2023-05-01 15:03:07 |
| Message-ID: | CA+TgmoamGjtqp_L5r5tMtrRbLA9durCesQjRBij3+H3tts7C5A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Apr 26, 2023 at 1:58 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> Why do we call wraparound wraparound, anyway? The 32-bit XID space is
> circular! The whole point of the design is that unsigned integer
> wraparound is meaningless -- there isn't really a point in "the
> circle" that you should think of as the start point or end point.
> (We're probably stuck with the term "wraparound" for now, so I'm not
> proposing that it be changed here, purely on pragmatic grounds.)
To me, the fact that the XID space is circular is the whole point of
talking about wraparound. If the XID space were non-circular, it could
never try to reuse the XID values that have previously been used, and
this entire class of problems would go away. Because it is circular,
it's possible for the XID counter to arrive back at a place that it's
been before i.e. it can wrap around.
--
Robert Haas
EDB: http://www.enterprisedb.com
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