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Re: cheaper snapshots

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: cheaper snapshots
Date: 2011-07-28 13:03:29
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Lists: pgsql-hackers
On Thu, Jul 28, 2011 at 3:46 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> Sounds like the right set of thoughts to be having.


> If you do this, you must cover subtransactions and Hot Standby. Work
> in this area takes longer than you think when you take the
> complexities into account, as you must.

Right.  This would replace the KnownAssignedXids stuff (a non-trivial
project, I am sure).

> I think you should take the premise of making snapshots O(1) and look
> at all the ways of doing that. If you grab too early at a solution you
> may grab the wrong one.

Yeah, I'm just brainstorming at this point.  This is, I think, the
best idea of what I've come up with so far, but it's definitely not
the only approach.

> For example, another approach would be to use a shared hash table.
> Snapshots are O(1), committing is O(k), using the snapshot is O(logN).
> N can be kept small by regularly pruning the hash table. If we crash
> we lose the hash table - no matter. (I'm not suggesting this is
> better, just a different approach that should be judged across
> others).

Sorry, I'm having a hard time understanding what you are describing
here.  What would the keys and values in this hash table be, and what
do k and N refer to here?

> What I'm not sure in any of these ideas is how to derive a snapshot xmin.

That is a problem.  If we have to scan the ProcArray every time we
take a snapshot just to derive an xmin, we are kind of hosed.  One
thought I had is that we might be able to use a sort of sloppy xmin.
In other words, we keep a cached xmin, and have some heuristic where
we occasionally try to update it.  A snapshot with a too-old xmin
isn't wrong, just possibly slower.  But if xmin is only slightly stale
and xids can be tested relatively quickly, it might not matter very

Robert Haas
The Enterprise PostgreSQL Company

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