Re: Lowering the ever-growing heap->pd_lower

On Thu, Aug 5, 2021 at 6:28 AM Simon Riggs <simon.riggs@enterprisedb.com> wrote:
> Hmm, there is no information in WAL to describe the line pointers
> being truncated by PageTruncateLinePointerArray(). We just truncate
> every time we see a XLOG_HEAP2_VACUUM record and presume it does the
> same thing as the original change.
>
> If that is safe, then we don't need to put the truncation on a WAL
> record at all, we just truncate after every XLOG_HEAP2_PRUNE record.

I agree that that's how we'd do it. This approach is no different to
assuming that PageRepairFragmentation() reliably produces a final
defragmented page image deterministically when called after we prune.

These days we automatically verify assumptions like this via
wal_consistency_checking. It would absolutely be able to catch any
bugs in PageTruncateLinePointerArray(), since the truncate code path
has plenty of coverage within the regression tests.

-- 
Peter Geoghegan