Re: Hot Standby b-tree delete records review

Simon Riggs wrote:
> On Thu, 2010-04-22 at 11:28 +0300, Heikki Linnakangas wrote:
>> Simon Riggs wrote:
>>> On Thu, 2010-04-22 at 10:24 +0300, Heikki Linnakangas wrote:
>>>> btree_redo:
>>>>> /*
>>>>> * Note that if all heap tuples were LP_DEAD then we will be
>>>>> * returning InvalidTransactionId here. This seems very unlikely
>>>>> * in practice.
>>>>> */
>>>> If none of the removed heap tuples were present anymore, we currently
>>>> return InvalidTransactionId, which kills/waits out all read-only
>>>> queries. But if none of the tuples were present anymore, the read-only
>>>> queries wouldn't have seen them anyway, so ISTM that we should treat
>>>> InvalidTransactionId return value as "we don't need to kill anyone".
>>> That's not the point. The tuples were not themselves the sole focus,
>> Yes, they were. We're replaying a b-tree deletion record, which removes
>> pointers to some heap tuples, making them unreachable to any read-only
>> queries. If any of them still need to be visible to read-only queries,
>> we have a conflict. But if all of the heap tuples are gone already,
>> removing the index pointers to them can'ẗ change the situation for any
>> query. If any of them should've been visible to a query, the damage was
>> done already by whoever pruned the heap tuples leaving just the
>> tombstone LP_DEAD item pointers (in the heap) behind.
>
> You're missing my point. Those tuples are indicators of what may lie
> elsewhere in the database, completely unreferenced by this WAL record.
> Just because these referenced tuples are gone doesn't imply that all
> tuple versions written by the as yet-unknown-xids are also gone. We
> can't infer anything about the whole database just from one small group
> of records.

Have you got an example of that?

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com