mapreduce - SELECT COUNT(DISTINCT *) in CouchDB -

i have documents complying format:

{   "_id": "some_doc_id",    "user": "some_user_id",    "date": "2015-09-15",   … } 

it's possible have multiple documents same user. i'd count how many distinct users there between 2 dates. e.g. between '2015-09-15' , '2015-09-25', there 745 different users.

in sql, write query:

select count(distinct user) documents date between '2015-09-15' , '2015-09-25'  

thank you.

i use map function like:

function (doc) {   emit(doc.date, doc.user); } 

which emit documents sorted date, user being value. reduce function this:

function(keys, values, rereduce) {   if (rereduce) {     return values.reduce(function (acc, index) {       return object.keys(index).reduce(function (acc, user) {         acc[user] = (acc[user] || 0) + index[user];         return acc;       }, acc);     }, {});   } else {     return values.reduce(function (acc, user) {       if (!(user in acc)) acc[user] = 0;       acc[user]++;       return acc;     }, {});   } } 

it's custom reduce function, bear me brief explanation. normal case (the 2nd branch, not rereduce) counts values finds. result object { some_user_id: 1 }.

the rereduce branch takes several of reduced objects , merges them (and counts) 1 reduced result. (you can read more reduce , rereduce here)

from there, can query view following query params:

start_key="2015-09-15" end_key="2015-09-25" 

you'll end same results shown earlier. (ie: { some_user_id: 1 }) on client, can count keys in resulting object idea how many unique users there given date range.