Re: Proposal for CSN based snapshots

On Tue, Jun 3, 2014 at 2:55 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> That doesn't address Bruce's concern about CLOG disk consumption.

Well we would only need the xid->lsn mapping for transactions since
globalxmin. Anything older we would just need the committed bit. So we
could maintain two structures, one like our current clog going back
until the freeze_max_age and one with 32-bits (or 64 bits?) per xid
but only going back as far as the globalxmin. There are a myriad of
compression techniques we could use on a sequence of mostly similar
mostly increasing numbers in a small range too. But I suspect they
wouldn't really be necessary.

Here's another thought. I don't see how to make this practical but it
would be quite convenient if it could be made so. The first time an
xid is looked up and is determined to be committed then all we'll ever
need in the future is the lsn. If we could replace the xid with the
LSN of the commit record right there in the page then future viewers
would be able to determine if it's visible without looking in the clog
or this new clog xid->lsn mapping. If that was the full LSN it would
never need to be frozen either. The problem with this is that the LSN
is big and actually moves faster than the xid. We could play the same
games with putting the lsn segment in the page header but it's
actually entirely feasible to have a snapshot that extends over
several segments. Way easier than a snapshot that extends over several
xid epochs. (This does make having a CSN like Oracle kind of tempting
after all)

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greg