I am trying to run a simple query to find number of all records with a particular value using:
db.ColName.find({id_c:1201}).count()
I have 200GB of data. When I run this query, mongodb takes up all the RAM and my system starts lagging. After an hour of futile waiting, I give up without getting any results.
What can be the issue here and how can I solve it?
I believe the right approach in the NoSQL world isn't trying to perform a full query like that, but accumulate stats overtime.
For example, you should have a collection stats with arbitrary objects which should own a kind or id property that can take a value like "totalUserCount". Whenever you add an user, you also update this count.
This way you'll get instant results. It's just getting a property value in a small collection of stats.
BTW, this slowness should be originated by querying objects by a non-indexed property in your collection. Try to index id_c and probably you'll get quicker results.