What is the CouchDB equivalent of the SQL COUNT(*) aggregate function?

Question

Yep, I'm a SQL jockey (sorta) coming into the CouchDb Map/Reduce world. I thought I had figured out how the equivalent of the COUNT(*) SQL aggregator function for CouchDB datasets with the following:

Map:

function(doc) {   emit(doc.name, doc); } 

Reduce:

function(keys, values, rereduce){   return values.length; } 

Which I thought worked, returning something like:

"super fun C"   2 "super fun D"   2 "super fun E"   2 "super fun F"   18 

... but not really. When I add a record, this count varies wildly. Sometimes the count actually decreases, which was very surprising. Am I doing something wrong? Maybe I don't fully understand the concept of eventual consistency?

Answer 1

In your reduce just put:

_count

You can also get a sum using:

_sum

so basically reduce: "_sum" or reduce: "_count" and make sure the value your map emits is a valid integer (numeric value)

See "Built in reduce functions".

Answer 2

It looks like your reduce results are being re-reduced. That is, reduce is called more than once for each key and then called again with those results. You can handle that with a reduce function like this:

function(keys, values, rereduce) {   if (rereduce) {     return sum(values);   } else {     return values.length;   } } 

Alternatively, you can change the map function so that the values are always a count of documents:

// map function(doc) {   emit(doc.name, 1); }  // reduce function(keys, values, rereduce) {   return sum(values); }