Re: Index Skip Scan

> On Thu, Jul 25, 2019 at 1:21 PM Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> I feel uncomfortable to look into indexinfo there. Couldnd't we
> use indexskipprefix == -1 to signal !amcanskip from
> create_index_path?

Looks like it's not that straightforward to do this only in create_index_path,
since to make this decision we need to have both parts, indexinfo and distinct
keys.

> Yeah, your explanation was perfect for me. What I failed to
> understand was what is expected to be done in the case. I
> reconsidered and understood that:
>
> For example, the following query:
>
> select distinct (a, b) a, b, c from t where c < 100;
>
> skip scan returns one tuple for one distinct set of (a, b) with
> arbitrary one of c, If the choosed c doesn't match the qual and
> there is any c that matches the qual, we miss that tuple.
>
> If this is correct, an explanation like the above might help.

Yes, that's correct, I've added this into commentaries.

> Maybe something like the following will work *for me*:p
>
> | When we are fetching a cursor in backward direction, return the
> | tuples that forward fetching should have returned. In other
> | words, we return the last scanned tuple in a DISTINCT set. Skip
> | to that tuple before returning the first tuple.

And this too (slightly rewritten:). We will soon post the new version of patch
with updates about UniqueKey from Jesper.