Re: Reviewing freeze map code

On Fri, Jun 24, 2016 at 4:33 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-06-23 18:59:57 -0400, Alvaro Herrera wrote:
>> Andres Freund wrote:
>>
>> > I'm looking into three approaches right now:
>> >
>> > 3) Use WAL logging for the already_marked = true case.
>>
>>
>> > 3) This approach so far seems the best. It's possible to reuse the
>> > xl_heap_lock record (in an afaics backwards compatible manner), and in
>> > most cases the overhead isn't that large. It's of course annoying to
>> > emit more WAL, but it's not that big an overhead compared to extending a
>> > file, or to toasting. It's also by far the simplest fix.
>>

+1 for proceeding with Approach-3.

>> I suppose it's fine if we crash midway from emitting this wal record and
>> the actual heap_update one, since the xmax will appear to come from an
>> aborted xid, right?
>
> Yea, that should be fine.
>
>
>> I agree that the overhead is probably negligible, considering that this
>> only happens when toast is invoked. It's probably not as great when the
>> new tuple goes to another page, though.
>
> I think it has to happen in both cases unfortunately. We could try to
> add some optimizations (e.g. only release lock & WAL log if the target
> page, via fsm, is before the current one), but I don't really want to go
> there in the back branches.
>

You are right, I think we can try such an optimization in Head and
that too if we see a performance hit with adding this new WAL in
heap_update.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com