Re: VACUUM (DISABLE_PAGE_SKIPPING on)

On Mon, Nov 16, 2020 at 1:52 PM Simon Riggs <simon(at)2ndquadrant(dot)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 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".
>

This patch seems mis-placed, at least in HEAD. If DISABLE_PAGE_SKIPPING
isn't doing what the documentation says it should, but instead provides
identical behavior to FREEZE, then the bug should be fixed in HEAD. I'd
argue for batch-patching it as well.

David J.