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MapReduce Compound Keys

Cloudant’s MapReduce mechansim is the original, and some say the best way of extracting data from a Cloudant database. In this blog post we’ll look at how compound keys can be used with MapReduce to provide flexible indexes for selection, range queries and grouped aggregation.

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How does MapReduce work?🔗

Let’s say we have documents like this:

  "_id": "myid",
  "date": "2023-08-18T10:24:59.000Z",
  "name": "Laura Jones",
  "email": "",
  "active": true,
  "score": 14.2

If we want to find active documents that match a given email we can create a MapReduce index with a JavaScript function like so:

functon(doc) {
  if ( {
    emit(, null)

This function is executed once for every document in the database and an index is created for every key/value pair emitted from a call to the emit function.


  • The if ensures that only active documents make it into the index. This makes the index smaller as inactive documents don’t make it to the index.
  • The emit function takes two parameters: the first is the key which defines the order of the index and a value (null in this case, which is returned with every row in the index at query-time).
  • We are using .toLowerCase() to ensure that the lower-cased version of our documents’ email addresses are added to the index. It is sometimes useful to have JavaScript available to pre-process the data prior to indexing.

This leads to an index with keys and values like so:

key value null null

We can then query the resultant index for a known key:

// find the first 50 documents whose email address equals ""
GET /mydb/_design/mydesigndoc/_view/myview?key=""&limit=50

Cloudant will return the matching rows in the index including the document’s _id, the index key and indexed value.

Using compound keys🔗

Instead of just indexing a single value (a string, number or boolea) we can instead index an array of values. Let’s say we want to index the year, month and day from our date field with this JavaScript map function:

function(doc) {
  if ( {
    const d = new Date(
    const k = [d.getFullYear(), d.getMonth() + 1, d.getDate()]
    const v = { name:, email: }
    emit(k, v)

Notice how we’re parsing the string as a JavaScript Date object and extracting the numeric year, month and day and indexing those as an array - with the most significant value to the left of the array. We’re also supplying a subset of the original document (its name and email) as the index’s “value”.

This leads to an index with keys and values like so:

key value
[2023,8, 18] { “name”: “Laura Jones”, “email”:“” }
[2023,8, 19] { “name”: “Bob Smith”, “email”:“” }

Like before we can query the index for a known value:

// find documents whose date is 18th August 2023
GET /mydb/_design/mydesigndoc/_view/myview?key=[2023,8,18]&limit=5
{"id":"0U3DQT2Q43UPA6J2","key":[2023,8,18],"value":{"name":"Noelia Greenberg","email":""}},
{"id":"1L4JIQCMX1JK0M77","key":[2023,8,18],"value":{"name":"Lennie Currie-Story","email":""}},
{"id":"27GTZEO3VRAFQ00M","key":[2023,8,18],"value":{"name":"Stanton Fuqua","email":""}},
{"id":"2O3BSLSLQG3V26BA","key":[2023,8,18],"value":{"name":"Micki Tolley","email":""}},
{"id":"2X37Y9ADBN69Q0NE","key":[2023,8,18],"value":{"name":"Columbus Brockman","email":""}}

As we have put some useful data into the value of the index, it is returned to us at query-time.

We can also supply startkey/endkey ranges on some or all of the key space:

// find documents whose date is in August 2023 (betweeen 1st Aug & 1st Sept)
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2023,8,1]&endkey=[2023,9,1]

We can omit some of the less-sigificant values:

// find documents whose date is between 2020 and 2024.
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2020]&endkey=[2024]

Compound keys and reducers🔗

The second component of MapReduce is “reduce”. At the time of creating the index we can specify one of the built-in reducer functions, which can be activated (or not) at query-time. The reducers are:

  • _count - for counting the number of indexed rows.
  • _sum - for totalising the values emitted.
  • _stats - for providing totals of the values emitted - useful for calculation of mean, variance and standard deviations.

The reduction phase is useful of aggregating and grouping data, as we can specify that we would like the count/sum/stats operation to be calculated on the one or more key values. Let’s look at an example. Using the Map function from the previous section, but with the _count reducer, our range query will now return the number of items on each day:

// return the number of items for each day in august.
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2023,8,1]&endkey=[2023,9,1]&group=true

By supplying the group_level we pick the granularity of keys that are used for grouping:

// return the number of items, grouped by year (first level of our [year, month, day] array)
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2020,1,1]&endkey=[2024,1,1]&group=true&group_level=1

// return the number of items, grouped by month (second level of our [year, month, day] array)
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2022,1,1]&endkey=[2023,1,1]&group=true&group_level=2

// return the number of items, grouped by day (third level of our [year, month, day] array)
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2023,1,1]&endkey=[2023,1,15]&group=true&group_level=3

We can also choose to switch the reducer off at query time to return matching rows, rather than totals:

// return individual rows between 2020 & the end of 2023.
GET /mydb/_design/mydesigndoc/_view/myview?startkey=[2020,1,1]&endkey=[2024,1,1]&reduce=false&limit=5
{"id":"0H1QUDL5IN3YVKYD","key":[2020,1,1],"value":{"name":"Isis Osgood","email":""}},
{"id":"2JUYUB4YS41DPV5E","key":[2020,1,1],"value":{"name":"Charita Timmerman","email":""}},
{"id":"4EXSQ77YXSEEUZDD","key":[2020,1,1],"value":{"name":"Dayna Hogan","email":""}},
{"id":"55IHT4U0N51FQRBQ","key":[2020,1,1],"value":{"name":"Rima Savage","email":""}},
{"id":"7EOEIHH911CQSPRV","key":[2020,1,1],"value":{"name":"Daria Reinhardt","email":""}}


Using compound keys as MapReduce keys in Cloudant produces secondary indexes that are good for:

  • Selection of matching known keys.
  • Selection between key ranges.
  • Aggregation with hierarchical grouping across the key space.