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From Łukasz Mielicki <mieli...@gmail.com>
Subject Re: reduce order vs descending
Date Wed, 23 Feb 2011 21:47:38 GMT
```On Wed, Feb 23, 2011 at 22:03, Paul Davis <paul.joseph.davis@gmail.com> wrote:

>> For: reduce (null, [key_range_A1_A2, key_range_B1_B2, key_range_C1_C2], true)
>> Does the relation A1 < A2 < B1 < B2 < C1 < C2 hold? Does it depend
on
>> descending?
>> For: reduce ([k1,k2,k3], [v1,v2,v3], false)
>> Can I depend on k1 < k2 < k3 if descending=true?

> These are specifically the assertions I'm saying you can't rely on. In
> your case you will absolutely need to sort the input data.

> The only assumption I think its possible to make is that re-reductions
> will always process reductions from contiguous parts of the b+tree.
> I'm only at the *think* stage of certainty. I haven't gone through the
> implementation to check if that's an absolute truth, and I can't yet
> guarantee that it won't ever change if it is true.
>
> Just in case that doesn't make sense, imagine you have a set of
> reductions that are from ordered regions of the view b+tree labeled as
> such: A, B, C, D. That is to say, data in the reduction A is based
> entirely of keys <= reduction keys in B <= reduction keys in C <=
> reduction keys in D. Given that, the only assumption I think is
> possible is that you are guaranteed that if A and C are passed
> simultaneously to a re-reduce, then B is guaranteed to be passed as
> well. In other words, you wont get a reduction graph that looks like
> ((A | C) | (B | D)) which would break this sort of approach.

Thanks for clarifying that.
The latter assumption was so inherent to me I didn't even consider it.
Don't think there would be much use of such reduce results at query
time.

However form bird's eye view basing on that one I cannot see a reason
not to preserve the first two (in terms of computational complexity
even with reduce parallelized). It seems to be only a matter of
preserving original b-tree partitioning in parameter passing.
Providing such guarantees would be beneficial in two ways:
- building a view would not depend on query parameters (sane)
- left/right-reduce semantics available (without sorting)

What you think?

Regards,
Łukasz

```
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