| From: | Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> |
|---|---|
| To: | Luca Ferrari <fluca1978(at)gmail(dot)com>, Jehan-Guillaume de Rorthais <jgdr(at)dalibo(dot)com> |
| Cc: | pgsql-general <pgsql-general(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: questions about wraparound |
| Date: | 2021-04-06 08:46:08 |
| Message-ID: | bcd5a091ce5abc8efa6790e3efb8ac8a0b52055c.camel@cybertec.at |
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| Lists: | pgsql-general |
On Sat, 2021-04-03 at 15:22 +0200, Luca Ferrari wrote:
> why having a TransactionId that is 32 bits
> in depth while it is exposed (thru txid_current()) as a 64 bits value?
> I mean, having 64 bits would reduce the need for anti-wrap arpund
> vacuum. I suspect the usage of 32 bits is both for compatibility and
> tuple header size, but I'm just guessing.
Because there are two of these transaction IDs stored on each tuple
(xmin and xmax) to determine its visibility. The overhead of 8 bytes
per tuples for visibility is already pretty high.
Another downside is that changing this would prevent the use of
pg_upgrade for upgrading, as the on-disk format changes.
Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com
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