Hi,
Actually you don't need more than the built-in _sum. consider;
map:
function(doc) {
emit(doc.key, [doc.foo, doc.bar, doc.baz]);
}
reduce:
_sum
this will sum each of the fields and return a 3-item array.
> On 13 Oct 2020, at 20:37, Jonathan Hall <[email protected]> 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 <[email protected]> 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.
>>>>
