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.
-- Simon Riggs www.2ndQuadrant.com
