[Standards] Exact hint for Result Set Management

Florian Schmaus flo at geekplace.eu
Thu Jul 12 14:03:30 UTC 2018


On 12.07.2018 15:20, Matthew Wild wrote:
> On 12 July 2018 at 13:13, Florian Schmaus <flo at geekplace.eu> wrote:
>> I'm well aware why RSM allows inexact results. That is not what this PR
>> is about.
>>
>> I simply think it is an oversight that RSM does not signal if the
>> results are exact or not.
> 
> Then I think you didn't quite consider my points?

I hope I did, sorry if not. It appears you consider two concepts the
same which I don't consider to be equal:
- Returning the exact result at the point in time, versus returning just
  an approximation.
- The possibility that the data changes in the future.


> The count can change at any time. So it may have been exact at the
> time of the query, but even by the time the client receives the count
> across the wire that count could be incorrect. What use is it for the
> client to know that the count was "exact" at some unknown point in
> recent history?

Possibly not much for those cases.

But there are cases where it comes in
useful for the client to know that the numbers are exact: For example
when syncing data.

C: How many entries are between X and Y?
S: Exactly 500.
C: Great that is my view of the data too and I know that the entries
between X and Y can not change, hence I know that my data is in sync.

vs.

C: How many entries are between X and Y?
S: Approximate 500.
Now C sees that the information is useless even if it also has 500
entries between X and Y, because the count given by S is (possibly)
approximate. So C goes home dealing with his grief (probably with the
help of a bottle scotch).

- Florian

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