Question

Point out the wrong statement.

(a) The query scans the index and not the collection

(b) If overall selectivity is high, and if MongoDB must read a number of documents to return results, then some queries may perform faster without indexes

(c) A better strategy, depending on your queries, would be to create a compound index that includes the high-selectivity field and another field

(d) None of the mentioned

I got this question in my homework.

This question is from Indexing Methods in chapter Index/Indexing Concepts of MongoDB

Answer 1

Right answer is (b) If overall selectivity is high, and if MongoDB must read a number of documents to return results, then some queries may perform faster without indexes

To explain I would say: To ensure selectivity, write queries that limit the number of possible documents with the indexed field. Write queries that are appropriately selective relative to your indexed data.