Re: getting rid of freezing

On 24 May 2013 17:00, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, May 24, 2013 at 11:29 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Fri, May 24, 2013 at 10:53 AM, Andres Freund <andres@2ndquadrant.com> wrote:
>>>> [all-visible cannot restore hint bits without FPI because of torn pages]
>>>
>>> I haven't yet thought about this sufficiently yet. I think we might have
>>> a chance of working around this, let me ponder a bit.
>>
>> Yeah.  I too feel like there might be a solution.  But I don't know
>> have something specific in mind, yet anyway.
>
> One thought I had is that it might be beneficial to freeze when a page
> ceases to be all-visible, rather than when it becomes all-visible.
> Any operation that makes the page not-all-visible is going to emit an
> FPI anyway, so we don't have to worry about torn pages in that case.
> Under such a scheme, we'd have to enforce the rule that xmin and xmax
> are ignored for any page that is all-visible; and when a page ceases
> to be all-visible, we have to go back and really freeze the
> pre-existing tuples.  I think we might be able to use the existing
> all_visible_cleared/new_all_visible_cleared flags to trigger this
> behavior, without adding anything new to WAL at all.

I like the idea but it would mean we'd have to freeze in the
foreground path rather in a background path.

Have we given up on the double buffering idea to remove FPIs
completely? If we did that, then this wouldn't work.

Anyway, I take it the direction of this idea is that "we don't need a
separate freezemap, just use the vismap". That seems to be forcing
ideas down a particular route we may regret. I'd rather just keep
those things separate, even if we manage to merge the WAL actions for
most of the time.


Some other related thoughts:

ISTM that if we really care about keeping xids for debug purposes that
it could be a parameter. For the mainline, we just freeze blocks at
the same time we do page pruning.

I think the right way is actually to rethink and simplify all this
complexity of Freezing/Pruning/Hinting/Visibility

--Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services