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Re: Commits 8de72b and 5457a1 (COPY FREEZE)

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Commits 8de72b and 5457a1 (COPY FREEZE)
Date: 2012-12-07 01:31:09
Message-ID: 1354843869.4530.156.camel@sussancws0025 (view raw or flat)
Thread:
Lists: pgsql-hackers
On Thu, 2012-12-06 at 20:12 -0500, Stephen Frost wrote:
> The command is 'FREEZE', which sounded to me like the transaction ID
> would be set to FrozenXID, meaning that we wouldn't be able to tell if
> the inserting transaction was before or after ours...

Freezing does lose information, but I thought that this sub-thread was
about the HEAP_XMIN_COMMITTED optimization that was in the first version
of the commit but removed in a later commit. Setting HEAP_XMIN_COMMITTED
does not lose information.

> Your analysis of the hint bits themselves sounds reasonable but it seems
> independent of the issue regarding setting the actual transaction ID.

Upon re-reading, my last paragraph was worded a little too loosely.

"The interesting thing about HEAP_XMIN_COMMITTED is that it can be set
preemptively if we know that the transaction will actually commit (aside
from the visibility issues within the transaction)."

That should really be: "aside from the visibility issues before it does
commit".

Anyway, the HEAP_XMIN_COMMITTED loading optimizations require more
discussion, but I think they are worth pursuing. The simpler form is
when the table is created and loaded in the same transaction, but there
may be some more sophisticated approaches, as well.

Regards,
	Jeff Davis



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