Re: CURRENT OF causes an error when IndexOnlyScan is used

Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp> writes:
> On Mon, 12 Mar 2018 13:56:24 -0400
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I took a quick look at this, but I'm concerned about a couple of points:

> In addition, this patch fixes only nbtree indexes, but the simillar problem
> will occur also on GIST indexes which support index-only scan. If we resolve
> this bug by fixing index access methods, I think we need to fix all existing
> indexes that support index-only scan and also describe the specification in
> the documents, comments, or README, etc. for future indexes.

Yeah, that's a pretty good point.

>> At this point Yugo-san's original patch is starting to look more
>> attractive. It's still ugly, but at least it's not imposing a performance
>> cost on unrelated queries.

> I would like to elaborate my patch if needed and possible. Any suggestion
> would be appriciated.

I don't think there's much else to be done so far as the IndexOnlyScan
case is concerned. However, I notice that somebody's made
search_plan_tree accept ForeignScanState and CustomScanState as possible
matches for WHERE CURRENT OF, and I find that rather troubling. It seems
likely to me that both of those would have the same problem as
IndexOnlyScans, ie whatever they're returning is probably a virtual tuple
without any ctid field. So we'd get the same unfriendly failure as you
complained of originally.

I wonder whether it wouldn't be a good idea to provide a way for an FDW
or CustomScan provider to return a TID, or at least give a more polite
FEATURE_NOT_SUPPORTED error than what happens now. However, that seems
more like a new feature than a bug fix; certainly any extension of those
APIs is something we'd not back-patch.

In the meantime, we could fix execCurrent.c so that it throws
FEATURE_NOT_SUPPORTED rather than the current failure if the slot it's
looking at doesn't contain a physical tuple.

regards, tom lane