In mongo, When creating an index I am trying to figure out whether the following query would have an index on a) category_ids and status, OR b) category_ids, status and name???
Source.where(category_ids: [1,2,3], status: Status::ACTIVE).order_by(:name) # ((Ruby/Mongoid code))
Essentially, I am trying to figure out whether indexes should include the ORDER_BY columns? or only the WHERE clauses? Where could I read some more about this?
Yes, an index on thius particular query would be beneficial to the speed of the query. However there is one caveat here, the order of the index fields.
I have noticed you are using an $in there on category_ids. This link is particularly useful in understanding a little complexity which exists from using an $in with an index on the sort (or a sort in general in fact): http://blog.mongolab.com/2012/06/cardinal-ins/
Towards the end it gives you an indea of an optimal index order for your type of query:
The order of fields in an index should be:
First, fields on which you will query for exact values. Second, fields on which you will sort. Finally, fields on which you will query for a range of values.
For reference a couple of other helpful links are as follows:
These will help you get started on optimising your indexes and making them work for your queries.
