On Tue, Nov 26, 2024 at 1:55 PM Masahiko Sawada <sawada.m...@gmail.com> wrote: > > Just to be clear, do users want the number of updated VM bits or the > number of pages whose visibility information is updated? For example, > > > visibility map: 5 pages set all-visible, 4 pages set all-frozen. > > IIUC the above log can be interpreted in two ways in terms of the > number of pages: > > (a) 5 pages are marked as all-visible, and other 4 > (already-marked-as-all-visible) pages are marked as all-frozen. That > is, 9 VM bits for 9 pages in total got updated. > (b) 1 page is marked as all-visible, and other 4 pages are marked as > all-frozen (and all-visible as well). That is, 9 VM bits for 5 pages > in total got updated. > > If users want to know "how many VM bits were updated?", the above log > makes sense. But there is no clear answer to "How many pages were > updated in terms of VM?".
Ah, good point. With a spin on the earlier example: create table foo (a int, b int) with (autovacuum_enabled = false); insert into foo select generate_series(1,1000), 1; delete from foo where a > 500; vacuum (verbose) foo; visibility map: 5 pages set all-visible, 2 pages set all-frozen. 5 pages were set all-visible and, of those, 2 were set all-frozen. So, 3 were set only all-visible. This is like (b) in your description. However, now if we do: vacuum (verbose, freeze) foo; visibility map: 0 pages set all-visible, 3 pages set all-frozen. Here, 3 already all-visible pages were set all-frozen. This does currently tell you the number of bits newly set, not the number of pages' whose VM status changed state. In fact, you could have a case where it is even more difficult to tell the total number of pages' whose VM status was updated. Let's say the first vacuum sets 5 pages newly all-visible, and of those, 2 are set all-frozen. Separately, 2 all-visible pages elsewhere in the relation are scanned (say due to SKIP_PAGES_THRESHOLD) and are old enough to require freezing. The message would be: visibility map: 5 pages set all-visible, 4 pages set all-frozen. But, we know 2 pages were set all-visible and all-frozen, 3 were set only all-visible, and 2 all-visible pages were set all-frozen. That's seven pages changing state. You would have no idea how many total pages changed state from the log message. So, since the transitions that are possible here are: nothing -> all-visible nothing -> all-visible and all-frozen all-visible -> all-visible and all-frozen What if we changed the message to reflect these state changes: visibility map: 5 pages newly set all-visible, of which 2 set all-frozen. 2 all-visible pages newly set all-frozen. - Melanie
