Re: Index-only scans and non-MVCC snapshots

On 27/06/2014 8:20 AM, Alvaro Herrera wrote:
> Ryan Johnson wrote:
>> On 26/06/2014 11:04 PM, Alvaro Herrera wrote:
>>> Ryan Johnson wrote:
>>>> As part of a research project, I'm trying to change Read Committed
>>>> isolation to use HeapTupleSatisfiesNow rather than acquiring a new
>>>> snapshot at every command [1].
>>> Are you aware of this?
>>>
>>> commit 813fb0315587d32e3b77af1051a0ef517d187763
>>> Author: Robert Haas <rhaas(at)postgresql(dot)org>
>>> Date: Thu Aug 1 10:46:19 2013 -0400
>>>
>>> Remove SnapshotNow and HeapTupleSatisfiesNow.
>> That would be wonderful news... if snapshots weren't so darned
>> expensive to create.
> I take it you aren't aware of this other effort, either:
> http://archives.postgresql.org/message-id/539AD153.9000004@vmware.com
That is good news, though from reading the thread it sounds like proc
array accesses are being exchanged for accesses to an SLRU, so a lot of
lwlock calls will remain. It will definitely help, though. SLRU will get
ex-locked a lot less often, so the main source of contention will be for
the actual lwlock acquire/release operations.

Regards,
Ryan