Re: MDAM techniques and Index Skip Scan patch

Dmitry Dolgov <9erthalion6(at)gmail(dot)com> writes:
> On Tue, Mar 22, 2022 at 04:55:49PM -0400, Tom Lane wrote:
>> In short: I would throw out just about all the planner infrastructure
>> that's been proposed so far. It looks bulky, expensive, and
>> drastically undercommented, and I don't think it's buying us anything
>> of commensurate value.

> Broadly speaking planner related changes proposed in the patch so far
> are: UniqueKey, taken from the neighbour thread about select distinct;
> list of uniquekeys to actually pass information about the specified
> loose scan prefix into nbtree; some verification logic to prevent
> applying skipping when it's not supported. I can imagine taking out
> UniqueKeys and passing loose scan prefix in some other form (the other
> parts seems to be essential) -- is that what you mean?

My point is that for pure loose scans --- that is, just optimizing a scan,
not doing AM-based duplicate-row-elimination --- you do not need to pass
any new data to btree at all. It can infer what to do on the basis of the
set of index quals it's handed.

The bigger picture here is that I think the reason this patch series has
failed to progress is that it's too scattershot. You need to pick a
minimum committable feature and get that done, and then you can move on
to the next part. I think the minimum committable feature is loose scans,
which will require a fair amount of work in access/nbtree/ but very little
new planner code, and will be highly useful in their own right even if we
never do anything more.

In general I feel that the UniqueKey code is a solution looking for a
problem, and that treating it as the core of the patchset is a mistake.
We should be driving this work off of what nbtree needs to make progress,
and not building more infrastructure elsewhere than we have to. Maybe
we'll end up with something that looks like UniqueKeys, but I'm far from
convinced of that.

regards, tom lane