Re: CURRENT OF causes an error when IndexOnlyScan is used

On Mon, 12 Mar 2018 13:56:24 -0400
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru> writes:
> > [ return_heaptuple_in_btree_indexonlyscan_v2.patch ]
>
> I took a quick look at this, but I'm concerned about a couple of points:
>
> 1. What's the performance penalty of forming (and then deforming) the
> added heap tuple? We'd be paying it for every index-only scan, whether
> or not any CURRENT OF fetch ever happened.
>
> 2. The constructed tuple provides tableoid and ctid all right, but it'd
> still have garbage values for other system columns. As the code stands,
> we will properly error out if some attempt is made to fetch any of those
> other columns, but with this change we'd just return the garbage. This
> is a minor point, but it doesn't seem negligible to me; it might've been
> hard to identify the bug at hand if we'd not had the cross-check of not
> building a heap tuple.

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.

> 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.

Thanks,

>
> regards, tom lane

--
Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>