Re: Lowering the ever-growing heap->pd_lower

Mark Dilger <mark(dot)dilger(at)enterprisedb(dot)com> writes:
>> On Mar 9, 2021, at 1:35 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> So, to accept a patch that shortens the line pointer array, what we need
>> to do is verify that every such code path checks for an out-of-range
>> offset before trying to fetch the target line pointer.

> Much as Pavan asked [1], I'm curious how we wouldn't already be in trouble if such code exists? In such a scenario, what stops a dead line pointer from being reused (rather than garbage collected by this patch) prior to such hypothetical code using an outdated TID?

The line pointer very well *could* be re-used before the in-flight
reference gets to it. That's okay though, because whatever tuple now
occupies the TID would have to have xmin too new to match the snapshot
that such a reference is scanning with.

(Back when we had non-MVCC snapshots to contend with, a bunch of
additional arm-waving was needed to argue that such situations were
safe. Possibly the proposed change wouldn't have flown back then.)

regards, tom lane