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 <vatam...@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 <fli...@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 <fli...@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.
>>>>>
