Re: XMAX_LOCK_ONLY and XMAX_COMMITTED (fk/multixact code)

> On Nov 24, 2021, at 12:53 PM, Bossart, Nathan <bossartn(at)amazon(dot)com> wrote:
>
> Another option we might consider is only checking for the
> HEAP_XMAX_LOCK_ONLY bit instead of everything in
> HEAP_XMAX_IS_LOCKED_ONLY. IIUC everything else is only expected to
> happen for upgrades from v9.2 or earlier, so it might be pretty rare
> at this point. Otherwise, I'll extract the exact bit pattern for the
> error message as you suggest.

I would prefer to detect and report any "can't happen" bit patterns without regard for how likely the pattern may be. The difficulty is in proving that a bit pattern is disallowed. Just because you can't find a code path in the current code base that would create a pattern doesn't mean it won't have legitimately been created by some past release or upgrade path. As such, any prohibitions explicitly in the backend, such as Asserts around a condition, are really valuable. You can know that the pattern is disallowed, since the server would Assert on it if encountered.

Aside from that, I don't really buy the argument that databases upgraded from v9.2 or earlier are rare. Even if servers *running* v9.2 or earlier are (or become) rare, servers initialized that far back which have been upgraded one or more times since then may be common.

—
Mark Dilger
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