On 7 December 2012 23:51, Stephen Frost <sfrost@snowman.net> wrote:
> Jeff,
>
> * Jeff Davis (pgsql@j-davis.com) wrote:
>> Most of your concerns seem to be related to freezing, and I'm mostly
>> interested in HEAP_XMIN_COMMITTED optimizations. So I think we're
>> talking past each other.
>
> My concern is about this patch/capability as a whole. I agree that the
> hint bit setting may be fine by itself, I'm not terribly concerned with
> that. Now, if you (and others) aren't concerned about the overall
> behavior that is being introduced here, that's fine, but it was my
> understanding from previous discussions that making tuples visible to
> all transactions, even those started before the committing transaction
> which are set more strictly than 'read-committed', wasn't 'ok'.
>
> Now, what I've honestly been hoping for on this thread was for someone
> to speak up and point out why I'm wrong about this concern and explain
> how this patch addresses that issue. If that's been done, I've missed
> it..
Visibility of pre-hinted data is an issue and because of that we
previously agreed that it would be allowed only when explicitly
requested by the user, which has been implemented as the FREEZE option
on COPY. This then makes it identical to TRUNCATE, where a separate
command differentiates MVCC-compliant row removal (DELETE) from
non-MVCC row removal (TRUNCATE).
So the committed feature does address the visibility issue. Jeff has
been speaking about an extension to that behaviour, which would be to
allow HEAP_XMIN_COMMITTED to be set in some cases also. The committed
feature specifically does not do that.
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services