Why does mongodb not use index scan but collection scan with find()?

Question

I am using mongodb 3.2.4

When I execute db.mytable.find().explain() The winning plan is 'Collscan'

But when I execute db.mytable.find().hint(_id:1).explain() The winning plan is 'IXscan'

So here comes a question: since _id is the default index of a table, why mongodb does not use this index to query?

Answer 1

An index can be used when there is a filter criteria or a sort operation - when the fields in the index are used in the filter predicate and/or the sort. In your case, the find method doesn't have a filter criteria or a sort - so no index is used, and you can see that in the query plan as a collection scan. It is as expected . But, when you provide a hint to the find method the query optimizer tries to use the index, and in your case it did (and you see it in the query plan as an IXSCAN). In either case, with or the without the hint, the find has to scan all the documents or keys in the index.

The _id has a default unique index, yes, but unless you are using the _id field in the query filter predicate or in a sort, the query cannot use it (or, specify explicitly to use index with a hint ). You can verify with the following queries, db.mytable.find( { _id: 123 } ) or db.mytable.find( { } ).sort( { _id: -1 } ) the query planner will show index scan even though you do not specify the hint .

The main purpose of the indexes is to make your queries run fast; it is about query performance. It has to be a query with filter predicate and/or a sort operation to use an index (and the fields used in the filter or sort must be indexed for performance). With the find method, in your case, without any of the two you are just accessing all the documents as they are in the collection and the index is of no use (and the query optimizer shows that in the plan).