On 4 December 2015 at 06:41, Robert Haas <robertmh...@gmail.com> wrote:

> I think there are ways to reduce the cost of this.  Some distributed
> systems have solved it by retreating from snapshot isolation and going
> back to using locks.  This can improve scalability if you've got lots
> of transactions but they're very short and rarely touch the same data.
> Locking individual data elements (or partitions) doesn't require a
> central system that knows about all commits - each individual node can
> just release locks for each transaction as it completes.  More
> generally, if you could avoid having to make a decision about whether
> transaction A precedes or follows transaction B unless transaction A
> and B actually touch the same data - an idea we already use for SSI -
> then you could get much better scalability.  But I doubt that can be
> made to work without a deeper rethink of our transaction system.

Something I've seen thrown about is the idea of lazy snapshots. Using
SSI-like facilities you keep track of transaction dependencies and what's
changed since the last snapshot. A transaction can use an old snapshot if
it doesn't touch anything changed since the snapshot was taken. If it does
you acquire a new snapshot (or switch to a newer existing one) and can swap
it in safely, since you know they're the same for all data the xact has
touched so far.

I suspect that just replaces one expensive problem with one or more
different expensive problems and is something that'd help a few workloads
but hurt others. It's come up when people have asked why we take a new
snapshot for every transaction but I haven't seen it explored much.

 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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