Re: SSI and Hot Standby

On Fri, Jan 21, 2011 at 10:32 AM, Tom Lane  wrote:
> Simon Riggs  writes:
>> On Fri, 2011-01-21 at 11:19 +0200, Heikki Linnakangas wrote:
>>> It's not the order in which the xid was assigned that matters, but the
>>> order the transactions started and got their snapshots. The xids might
>>> be assigned a lot later, after the transactions have already read data.
>
>> So if a read-write transaction assigns an xid before it takes a snapshot
>> then we'll be OK? That seems much easier to arrange than passing chunks
>> of snapshot data backwards and forwards. Optionally.
>
> No, that idea is DOA from a performance standpoint.  We sweated blood to
> avoid having to assign XIDs to read-only transactions, and we're not
> going back.  If SSI requires that, SSI is not getting committed.

So far I think all of the ideas proposed for generalizing this across
the master-standby connection seem likely to be DOA from a performance
perspective.  But I think we have a pretty broad consensus that it's
OK to punt this issue for 9.1.  We can always add this in 9.2 if it
can be demonstrated to work well, but it's all vapor-ware right now
anyway.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company