Count distinct values in mongoDB

Question

This is the document structure in mongoDB

{ "_id" :ObjectId("9elesdf3lk3jppefll34d210"), "category" :"data1",product:'data'}
{ "_id" :ObjectId("9elesdf3lk3jppefll34d211"), "category" : "data2",product:'data'}
{ "_id" :ObjectId("9elesdf3lk3jppefll34d211"), "category" : "data1",product:'data' }

where category is indexed. I want to take a distinct count of the category field.

Currently I am using the following code to take the counts

db.collection.aggregate( 
   {$group : {_id : "$category"} }, 
   {$group: {_id:1, count: {$sum : 1 }}})

This query was giving me proper counts but my database is increasing day by day and the query is taking longer to execute. Is there some other methodology to take the counts in a faster way?

Have you tested the performance of db.collection.distinct('category').length as an alternative? distinct can use an index but $group cannot. — JohnnyHK
– JohnnyHK, Commented Jul 24, 2016 at 4:01
@JohnnyHK i have more than 10 million documents and growing..will it be able to procees so much data at once? — mikhil mohanan
– mikhil mohanan, Commented Jul 24, 2016 at 6:39
As long as it's able to use an index, sure. Roughly how many different categories are there? — JohnnyHK
– JohnnyHK, Commented Jul 24, 2016 at 13:20
@mikhilmohanan You should start accepting answers when you want help from SO users in the future, as there are 8 questions from you out there with answers but you didn't accept any of them ... — DAXaholic
– DAXaholic, Commented Jul 29, 2016 at 4:34
@DAXaholic Thankyou for oyur comment.ill take care of it in the future. — mikhil mohanan
– mikhil mohanan, Commented Aug 1, 2016 at 13:41

Community · Accepted Answer · 2017-05-23 12:14:56Z

3

As already pointed out by JohnnyHK, use db.collection.distinct if possible as it provides the chance of leveraging indexes

So in your case db.collection.distinct('category').length should be pretty fast.
If you still suffer from performance issues then have a look at

db.collection.explain().distinct('category')

to see the execution plan of the query and take actions on it or provide it to this question so that we see whether your index is actually used.

edited May 23, 2017 at 12:14

CommunityBot

11 silver badge

answered Jul 24, 2016 at 5:07

DAXaholic

35.9k6 gold badges84 silver badges77 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

mikhil mohanan Over a year ago

i have executed the query..can you help me in identity which field specify that my index is being used or not

DAXaholic Over a year ago

If you execute the ..explain().distinct('category') then you should see somewhere under queryPlanner.winningPlan a 'DISTINCT_SCAN' stage. If you see a 'COLLSCAN' stage then your index is not used.

Collectives™ on Stack Overflow

Count distinct values in mongoDB

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related