Mongodb mapreduce sorting (optimization) or alternative - mongodb

Question

I have a few documents that look like this:
{
'page_id': 123131,
'timestamp': ISODate('2014-06-10T12:13:59'),
'processed': false
}
The documents have other fields, but these are the only one relevant for this purpose. On this collection is also an index for these documents:
{
'page_id': 1
'timestamp': -1
}
I run a mapreduce that returns distinct (page_id, day) results, with day being the date-portion of the timestamp (in the above, it would be 2014-06-10).
This is done with the following mapreduce:
function() {
emit({
site_id: this.page_id,
day: Date.UTC(this.timestamp.getUTCFullYear(),
this.timestamp.getUTCMonth(),
this.timestamp.getUTCDate())
}, {
count: 1
});
}
The reduce-function basically just returns { count: 1 } as I am not really interested in the number, just unique tuples.
I wish to make this more efficient. I tried adding sort: { 'page_id' }, but it triggers an error - googling shows that I can apparently only sort by the key, but since this is not a "raw" key how does that work?
Also, is there an alternative to this mapreduce that is faster? I know mongodb has the distinct, but from what I can gather it only works on one field. Might the group aggregate function be relevant?

Answer 1

The aggregation framework would seem more appropriate since it runs in native code where mapReduce runs under a JavaScript interpreter instance. MapReduce has it's uses, but generally the aggregation framework should be best suited to common tasks that do not require specific processing where only the JavaScript methods allow the needed control:
db.collection.aggregate([
{ "$group": {
"_id": {
"page": "$page_id",
"day": {
"year": { "$year": "$timestamp" },
"month": { "$month": "$timestamp" },
"day": { "$dayOfMonth": "$timestamp" },
}
},
"count": { "$sum": 1 }
}}
])
This largely makes use of the date aggregation operators. See other aggregation framework operators for more details.
Of course if you wanted to reverse sort those unique dates (which is the opposite of what mapReduce will do) or other fields then just add a $sort to the end of the pipeline for what you want:
db.collection.aggregate([
{ "$group": {
"_id": {
"page": "$page_id",
"day": {
"year": { "$year": "$timestamp" },
"month": { "$month": "$timestamp" },
"day": { "$dayOfMonth": "$timestamp" },
}
},
"count": { "$sum": 1 }
}},
{ "$sort": {
"day.year": -1, "day.month": -1, "day.day": -1
}}
])

Answer 2

you might want to look at the aggregation framework.
query like this:
collection.aggregate([
{$group:
{
_id: {
year: { $year: [ "$timestamp" ] },
month: { $month: [ "$timestamp" ] },
day: { $dayOfMonth: [ "$timestamp" ] },
pageId: "$page_id"
}
}
])
will give you all unique combinations of the fields you're looking for.