On Tue, Nov 17, 2020 at 5:52 AM Simon Riggs <si...@2ndquadrant.com> wrote: > > The docs are misleading for this feature, since they say: > "This option disables all page-skipping behavior, and is > intended to be used only when the contents of the visibility map are > suspect, which should happen only if there is a hardware or software > issue causing database corruption." > > The docs do correctly say "Pages where all tuples are known to be > frozen can always be skipped". Checking the code, lazy_scan_heap() > comments say > "we can still skip pages that are all-frozen, since such pages do not > need freezing". > > The code is quite clear: DISABLE_PAGE_SKIPPING makes the vacuum into > an aggressive vacuum. Line 487, heap_vacuum_rel(). Aggressive vacuums > can still skip a page that is frozen, and rely on the visibility map > for that information. > > So the docs are wrong - we don't disable *all* page-skipping and it is > not appropriate to warn users away from this feature by saying "is > intended to be used only when the contents of the visibility map are > suspect".
I don't think the doc is wrong. If DISABLE_PAGE_SKIPPING is specified, we not only set aggressive = true but also skip checking visibility map. For instance, see line 905 and line 963, lazy_scan_heap(). Regards, -- Masahiko Sawada EnterpriseDB: https://www.enterprisedb.com/
