CouchDB: getting number of keys in given key range - couchdb

Question

In my CouchDB database, all keys have the form "A_xxxxxxxx" where xxxxxxxx is zero-padded decimal number (e.g. "A_00000001" or "A_12345678")
I want to get only the number of keys in a given key range.
For example, to get the keys from A_10000000 to A_30000000, I can query something like:
GET DATABASE/_all_docs?startkey="A_00001000"&endkey="A_30000000"&include_docs=false
But the result contains all keys, and I need to count the elements in "docs" field of the output.
Since the number of keys in my query will be huge, and all I want to know is the number of keys, not the actual list of the keys.
The range start and range end value can be vary, which is not fixed.
Is is possible to get only the number of keys of the given range, without retrieving actual key list?
Thanks,

Answer 1

You cannot get the number of keys in a given key range using the built-in _all_docs view. But you can get the desired result using a custom map reduce view such as this one described in the CouchDB Definitive Guide
map.js
function(doc) {
emit(doc._id, 1);
}
reduce.js
function(keys, values, rereduce) {
return sum(values)
}
You can add these views to your CouchDB database using the Futon admin utility by creating a new document with these contents:
{
"_id": "_design/test",
"views": {
"count": {
"map": "function(doc) {\n emit(doc._id, 1);\n}",
"reduce": "function(keys, values, rereduce) {\n return sum(values)\n}"
}
}
}
_design/test/count can then be queried like instead of _all_docs and will return the number of documents between the start and end keys.
When I run this query again my database without a start and end key I get this result:
{
"rows":[
{
"key": null,
"value": 185
}
]
}
Running the query again with the start and end keys populated I get this result:
{
"rows":[
{
"key": null,
"value": 11
}
]
}