Index Vs Query Criteria

Question

I am new to MongoDB. I have read that Indexes limit the documents to be scanned when we query for some data. Reference:-http://docs.mongodb.org/manual/core/indexes-introduction I am confusing it with the Query Criteria as it limits the data. For example:- db.users.find({score:{"$lt":30}}). In the manual this example is given and explained in the sense of indexes. What are indexes and how are they different than Query Criteria? Thank you

http://stackoverflow.com/questions/1108/how-does-database-indexing-work this is what you need — Sammaye, Jan 30 '14 at 11:30

score 0 · Answer 1 · answered Jan 30 '14 at 15:13

Indexes in MongoDB are similar, but not identical to indexes in relational databases. So, to get a basic feel, you can think of the two. Query criteria defines what subset of documents your queries are interested in. An index may be able to USE the query criteria to answer the query faster.

Suppose you have a collection with no indexes, and you do db.users.find({score:{$lt:30}}). With no index, you will need to scan the entire collection to answer the query, processing all documents regardless of their value. With an index on 'score', the query will be able to use the index to drill down on ONLY the documents that match your query, thereby executing faster.

score 0 · Accepted Answer · answered Jan 30 '14 at 16:01

Query criteria limits the data that is send to the client from server but it has to scan each and every document for the matching. On the other hand Index limits the scanning of the documents by having special data structure(B-tree in mongodb). Ref:-http://docs.mongodb.org/manual/core/indexes-introduction

Index Vs Query Criteria

2 Answers2