Re: cheaper snapshots

On Thu, 2011-07-28 at 18:05 +0200, Hannu Krosing wrote:

> But it is also possible, that you can get logically consistent snapshots
> by protecting only some ops. for example, if you protect only insert and
> get snapshot, then the worst that can happen is that you get a snapshot
> that is a few commits older than what youd get with full locking and it
> may well be ok for all real uses.

Thinking more of it, we should lock commit/remove_txid and get_snapshot

having a few more running backends does not make a difference, but
seeing commits in wrong order may.

this will cause contention between commit and get_snapshot, but
hopefully less than current ProcArray manipulation, as there is just one
simple C array to lock and copy.

-- 
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Hannu Krosing
PostgreSQL Infinite Scalability and Performance Consultant
PG Admin Book: http://www.2ndQuadrant.com/books/