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From Nick Vatamaniuc <vatam...@gmail.com>
Subject Re: [DISCUSS] Deprecate custom reduce functions
Date Tue, 13 Oct 2020 20:39:52 GMT
Makes sense. I am +1 to #2 (disable by default), and +0 neutral on #1
(deprecate)

On Tue, Oct 13, 2020 at 4:21 PM Robert Samuel Newson <rnewson@apache.org> wrote:
>
>
> Nick, let's broaden the thread to two questions then;
>
> 1) Deprecate custom reduce functions
> 2) Disable custom reduce functions by default, but don't deprecate them.
>
>
>
> > On 13 Oct 2020, at 21:16, Nick Vatamaniuc <vatamane@gmail.com> wrote:
> >
> > In case of _sum, like Joan mentioned, we can emit objects or arrays
> > and the built-in _sum will add the values of the fields together:
> >
> > So  {"map": 'function(d){ emit(d._id, {"bar":1, "foo":2, "baz":3});
> > }',  "reduce" : '_sum' } for 10 docs would produce {"bar": 10, "baz":
> > 30, "foo": 20}.
> >
> > As for the deprecation, I wouldn't necessarily call for deprecation
> > but I can see leaving it disabled by default and let the users enable
> > it if they want to. If we see that there is a good demand for custom
> > functions, and it is annoying for users to have to enable it, we could
> > revert it back to enabled by default or like it was discussed, or, try
> > to add more built-in reducers.
> >
> > Cheers,
> > -Nick
> >
> > On Tue, Oct 13, 2020 at 3:38 PM Jonathan Hall <flimzy@flimzy.com> wrote:
> >>
> >> So looking through the code that uses this, it looks like the main use
> >> I've had for custom reduce functions is summing multiple values at
> >> once.  A rough equivalent of 'SELECT SUM(foo),SUM(bar),SUM(baz)'.
> >>
> >> The first thing that comes to mind to duplicate this functionality
> >> without a custom reduce function would mean building one unique index
> >> for each value that needs to be summed, which I expect would be a lot
> >> less efficient.
> >>
> >> But maybe I'm overlooking a more clever and efficient alternative.
> >>
> >> Jonathan
> >>
> >>
> >> On 10/13/20 6:31 PM, Robert Samuel Newson wrote:
> >>> Hi,
> >>>
> >>> Yes, that's what I'm referring to, the javascript reduce function.
> >>>
> >>> I'm curious what you do with custom reduce that isn't covered by the built-in
reduces?
> >>>
> >>> I also think if custom reduce was disabled by default that we would be motivated
to expand this set of built-in reduce functions.
> >>>
> >>> B.
> >>>
> >>>> On 13 Oct 2020, at 17:06, Jonathan Hall <flimzy@flimzy.com> wrote:
> >>>>
> >>>> To be clear, by "custom reduce functions" you mean this (https://docs.couchdb.org/en/stable/ddocs/ddocs.html#reduce-and-rereduce-functions)?
> >>>>
> >>>> So by default, only built-in reduce functions could be used (https://docs.couchdb.org/en/stable/ddocs/ddocs.html#built-in-reduce-functions)?
> >>>>
> >>>> If my understanding is correct, I guess I find it a but surprising.
I've always thought of map/reduce of one of the core features of CouchDB, so to see half of
that turned off (even if it can be re-enabled) makes me squint a bit. And it is a feature
I use, so I would not be in favor of deprecating it entirely, without a clear proposal/documentation
for an alternative/work-around.
> >>>>
> >>>> Based on the explanation below, it doesn't sound like there's a technical
reason to deprecate it, but rather a user-experience reason. Is this correct?
> >>>>
> >>>> If my understanding is correct, I'm not excited about the proposal,
but before I dive further into my thoughts, I'd like confirmation that I actually understand
the proposal, and am not worried about something else ;)
> >>>>
> >>>> Jonathan
> >>>>
> >>>>
> >>>> On 10/13/20 5:48 PM, Robert Samuel Newson wrote:
> >>>>> Hi All,
> >>>>>
> >>>>> As part of CouchDB 4.0, which moves the storage tier of CouchDB
into FoundationDB, we have struggled to reproduce the full map/reduce functionality. Happily
this has now happened, and that work is now merged to the couchdb main branch.
> >>>>>
> >>>>> This functionality includes the use of custom (javascript) reduce
functions. It is my experience that these are very often problematic, in that much more often
than not the functions do not significantly reduce the input parameters into a smaller result
(indeed, sometimes the output is the same or larger than the input).
> >>>>>
> >>>>> To that end, I'm asking if we should deprecate the feature entirely.
> >>>>>
> >>>>> In scope for this thread is the middle ground proposal that Paul
Davis has written up here;
> >>>>>
> >>>>> https://github.com/apache/couchdb/pull/3214
> >>>>>
> >>>>> Where custom reduces are not allowed by default but can be enabled.
> >>>>>
> >>>>> The core _ability_ to do custom reduces will always been maintained,
this is intrinsic to the design of ebtree, the structure we use on top of FoundationDB to
hold and maintain intermediate reduce values.
> >>>>>
> >>>>> My view is that we should merge #3214 and disable custom reduces
by default.
> >>>>>
> >>>>> B.
> >>>>>
>

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