Simon Riggs <[email protected]> writes:
> On 8 December 2012 15:14, Tom Lane <[email protected]> wrote:
>> Or we could wait for MVCC catalog access ...
> If there was a published design for that, it would help believe in it more.
> Do you think one exists?
Well, there have been discussion threads about it in the past. I don't
recall whether any insoluble issues were raised. I think the concerns
were mostly about performance, if we start taking many more snapshots
than we have in the past.
The basic idea isn't hard: anytime a catalog scan is requested with
SnapshotNow, replace that with a freshly taken MVCC snapshot. I think
we'd agreed that this could safely be optimized to "only take a new
snapshot if any new heavyweight lock has been acquired since the last
one". But that'll still be a lot of snapshots, and we know the
snapshot-getting code is a bottleneck already. I think the discussions
mostly veered off at this point into how to make snapshots cheaper.
regards, tom lane
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