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From: Tom Lane <[email protected]>
To: Egor Rogov <[email protected]>
Cc: [email protected]
Subject: Re: FrozenTransactionId
Date: Tue, 06 Sep 2016 08:41:34 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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Egor Rogov <[email protected]> writes:
>> Please find another patch attached. I hope it is a bit easier to 
>> review now.

Hmm.  I do not actually see anything wrong with the existing text:
it does not say that FrozenTransactionId is what appears on disk,
it says that frozen tuples are treated as if their xmin were
FrozenTransactionId.  That's an accurate description of the current
implementation.  I'm not sure that removing all mention of special
XIDs is a good thing, especially since they will still exist on-disk
in pg_upgrade'd installations.

It does seem like this paragraph could do with some copy-editing,
but I doubt that removing information is the way to go about that.
What I'd change:
1. Grammar fix: s/effects ... is/effects ... are/
2. Remove the useless phrase ", from an MVCC perspective,"
3. Reorder the sentences just after that, so that we have
"Normal XIDs are compared ... suddenly appear to be in the future.
To prevent this, PostgreSQL reserves a special XID, FrozenTransactionId,
... considered older than every normal XID.
Frozen row versions are treated as if ..."

It might also be worth mentioning explicitly that FrozenTransactionId
can appear on-disk in old installations.  And I'd be slightly tempted
to put in an explanation of BootstrapTransactionId, because users
will see that as well if they look into any system catalogs.

Comments?

			regards, tom lane


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