As I understand the reduce function takes all the values of a particular key and we can write code to perform some kind of action on those values. I do not understand what is the use of the rereduce parameter. Can somebody explain with an example?

Thanks...

As I understand the reduce function takes all the values of a particular key and we can write code to perform some kind of action on those values. I do not understand what is the use of the rereduce parameter. Can somebody explain with an example?

Thanks...

• javascript
• couchdb
• mapreduce

2 Answers 2

I think http://wiki.apache/couchdb/Introduction_to_CouchDB_views#Reduce_vs_rereduce gives you an good overview. Due to performance optimations, the reduce function may be called on two levels:

• to reduce a block
• to reduce results of the first step

In the second level, the parameter rereduce is true. For an example see http://wiki.apache/couchdb/Built-In_Reduce_Functions#A_sum. In the first step the length of the block (values) is returned, on the rereduce level these lengths must be summed up.

There's an explanation of the rereduce parameter here.

Quote: paraphrased. More info at the link.

Here is an example of a reduce function:

function (key, values, rereduce) {
    return sum(values); 
}

Reduce functions must handle two cases, when rereduce is true and when it is false.