Re: Amcheck verification of GiST and GIN

On Thu, Feb 2, 2023 at 11:51 AM Peter Geoghegan <pg@bowt.ie> wrote:
> I also have some questions about the verification functionality itself:

I forgot to include another big concern here:

* Why are there only WARNINGs, never ERRORs here?

It's far more likely that you'll run into problems when running
amcheck this way. I understand that the heapam checks can do that, but
that is both more useful, and less risky. With heapam we're not
traversing a tree structure in logical/keyspace order. I'm not
claiming that this approach is impossible; just that it doesn't seem
even remotely worth it. Indexes are never supposed to be corrupt, but
if they are corrupt the solution always involves a REINDEX. You never
try to recover the data from an index, since it's redundant and less
authoritative, almost by definition (at least in Postgres).

By far the most important piece of information is that an index has
some non-zero amount of corruption. Any amount of corruption is
supposed to be extremely surprising. It's kind of like if you see one
cockroach in your home. The problem is not that you have one cockroach
in your home; the problem is that you simply have cockroaches. We can
all agree that in some abstract sense, fewer cockroaches is better.
But that doesn't seem to have any practical relevance -- it's a purely
theoretical point. It doesn't really affect what you do about the
problem at that point.

Admittedly there is some value in seeing multiple WARNINGs to true
experts that are performing some kind of forensic analysis, but that
doesn't seem worth it to me -- I'm an expert, and I don't think that
I'd do it this way for any reason other than it being more convenient
as a way to get information about a system that I don't have access
to. Even then, I think that I'd probably have serious doubts about
most of the extra information that I'd get, since it might very well
be a downstream consequence of the same basic problem.

-- 
Peter Geoghegan