Re: Re: [COMMITTERS] pgsql: Reduce spurious Hot Standby conflicts from never-visible records

On Wed, Jan 5, 2011 at 3:00 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Mon, 2011-01-03 at 23:13 -0500, Robert Haas wrote:
>
>> > Hmmm, my earlier code took xmax only if xmax > xmin. That was wrong;
>> > what I have now is better, but your point is there may be an even better
>> > truth. I'll think on that a little more.
>
> I remember that I thought some more on this and decided that I couldn't
> see a problem. I also see I didn't update the list to say that.
>
>> I guess the problem case here is something like:
>>
>> 1. T1 begins.  T1 writes a tuple A (so it gets an XID).
>> 2. T2 begins.  T2 writes a tuple B (so it gets a later XID).
>> 3. T1 takes a new snapshot that can see B and deletes B.
>> 4. T2 commits.
>> 5. T1 commits.
>
> How is step (3) possible before step (4)?

At read committed isolation level, which is the default, we take a new
snapshot after every command.

> There are later errors in your example also.

Well, point them out and let's discuss.

--
Robert Haas
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