Question

I have a MongoDB collection with various data in it. (about millions)

These data have a data struct like {k: {a:1,b:2,c:{},...}} and I don’t know extactly what in it.

Now I wanna do a counting on this collection to return me the total elements in the collection that k is not empty by using {k:{$exists:true}} but that’s turns out very slow …

Then I add an index on k and trying to query by : {k:{$gt:{}} but that’s not return the correct results.

So, how to do this counting on the collection now?

Note that I don’t know the data structure of k.

Answer 1

db.collection.count({k:{$ne:null}})

By the way use sparse index on k.

db.collection.ensureIndex({k:1}, {sparse: true});