FAQ

On 7 June 2013 20:16, Andres Freund wrote:
On 2013-06-07 20:10:55 +0100, Simon Riggs wrote:
On 7 June 2013 19:56, Heikki Linnakangas wrote:
On 07.06.2013 21:33, Simon Riggs wrote:

Now that I consider Greg's line of thought, the idea we focused on
here was about avoiding freezing. But Greg makes me think that we may
also wish to look at allowing queries to run longer than one epoch as
well, if the epoch wrap time is likely to come down substantially.

To do that I think we'll need to hold epoch for relfrozenxid as well,
amongst other things.
The biggest problem I see with that is that if a snapshot can be older than
2 billion XIDs, it must be possible to store XIDs on the same page that are
more than 2 billion XIDs apart. All the discussed schemes where we store the
epoch at the page level, either explicitly or derived from the LSN, rely on
the fact that it's not currently necessary to do that. Currently, when one
XID on a page is older than 2 billion XIDs, that old XID can always be
replaced with FrozenXid, because there cannot be a snapshot old enough to
not see it.
It does seem that there are two problems: avoiding freezing AND long
running queries

The long running query problem hasn't ever been looked at, it seems,
until here and now.
I'd say that's because it's prohibitive to run so long transactions
anyway since it causes too much unremovable bloat. 2bio transactions
really is a quite a bit, I don't think it's a relevant restriction. Yet.

Let's discuss it if we have solved the other problems ;)
Let me say that I think that problem is solvable also.

At the moment we allow all visible tuple versions to be linked
together, so that the last visible and latest update are linked by a
chain. If we break that assumption and say that we will never follow
an update chain from a snapshot in the distant past, then we can
remove intermediate dead rows. We currently regard those as recently
dead, but that just requires some extra thought. We still keep all
*visible* tuple versions, we just don't bother to keep all the
intermediate ones as well.

Perhaps another day, but one day.

--
  Simon Riggs http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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