Hi,
On 2021-11-10 18:16:18 -0800, Andres Freund wrote:
> On 2021-11-10 17:37:38 -0800, Peter Geoghegan wrote:
> > You might also move the call to GlobalVisTestFor() out of
> > lazy_scan_heap(), so that it gets called right after
> > vacuum_set_xid_limits(). That would make the new explanation easier to
> > follow, since you are after all explaining the relationship between
> > OldestXmin (or the vacuum_set_xid_limits() call itself) and vistest
> > (or the GlobalVisTestFor() call itself).
> >
> > Why do they have to be called in that order? Or do they? I noticed
> > that "make check-world" won't break if you switch the order.
>
> We need pruning to be at least as aggressive as relfrozenxid. If we did it the
> other way round, we couldn't guarantee that.
There's a relevant comment above ComputeXidHorizons():
* Note: the approximate horizons (see definition of GlobalVisState) are
* updated by the computations done here. That's currently required for
* correctness and a small optimization. Without doing so it's possible that
* heap vacuum's call to heap_page_prune() uses a more conservative horizon
* than later when deciding which tuples can be removed - which the code
* doesn't expect (breaking HOT).
Greetings,
Andres Freund