This document discusses indexing and query optimization in MongoDB. It provides an overview of indexes and how they can improve query performance by allowing certain queries to use indexes rather than scanning all documents. It describes different types of indexes like compound, unique, multikey, covered and sparse indexes. It also covers topics like profiling queries, creating and managing indexes, and strategies for effective indexing.

1. MongoDB
Indexing & Query
Optimization

2. Overview
• Indexing
• Profiling your queries
• Creating indexes
• Managing indexes

3. What is an Index?
A data structure that can used to make certain
query more efficient ?

4. Table scans
1 2 3 4 5 6 7
Looked at 7 objects
Find where x equals 7

5. Tree Lookup
7
6
5
4
3
2
1
Looked at 3 objects
Find where x equals 7

6. Use explain
query = db.user.find({title:‛Blog‛})
query.explain();
{
"cursor" : "BasicCursor",
"indexBounds" : [ ],
"nscanned" : 57594,
"nscannedObjects" : 57594,
"n" : 3,
"millis" : 108
}

7. Creating Indexes

8. Index a field
db.user.ensureIndex( { ‘name’: 1 })
1 = ascending
-1 = descending
Test last query with explain method.

9. Compound indexes
db.posts.ensureIndex({name: 1, date: -1})
Indexes maintain order index{a:1}
{a: 0, b : 1}
{a: 1, b : 2}
{a: 1, b : 3}
{a: 2, b : 1}
{a: 3, b : 2}
{a: 3, b : 7}
{a: 3, b : 9}

10. index{a:1,b:-1}
{a: 0, b : 1}
{a: 1, b : 3}
{a: 1, b : 2}
{a: 2, b : 1}
{a: 3, b : 9}
{a: 3, b : 7}
{a: 3, b : 2}

11. Unique Indexes
db.user.ensureIndex({title: 1}, {unique: true})

12. Embedded documents
db.user.save({
title: ‚My First blog‛,
comments: [
{author: ‚Ram‛, time : new Date()} ]
});
db.posts.ensureIndex({‚comments.author‛: 1})

13. Multikeys
{ ‚tags‛:
[‚mongodb‛, ‚cool‛,‛balck]
, ...}
db.posts.ensureIndex({‚tags‛: 1})

14. Covered Indexes
• New in 1.7.4
• Query can be resolved in index only
• Need to exclude _id from items projected
db.user.ensureIndex({‚title‛: 1})
db.user.find({‚title‛: ‚My blog post:},
{title: 1, _id:0}))

15. Sparse Indexes
• Included if and only if indexed values is present
• Limited to single field
db.user.ensureIndex({ title : 1} , {sparse :true} )
db.user.save({ name : Rohan})
db.user.save({ name : Ram , title : princes })
Return only Ram
db.user.find ( { title : { $ne : null} } ) .sort({ title : 1} )

16. Geospatial Indexes

17. Managing Indexes

18. Listing indexes
db.user.getIndexes();
Dropping indexes
db.user.dropIndex({tag :1})
Background Building Indexes
db.user.ensureIndex( { … },{background : true} })

19. Indexing Strategies

20. Strategies
• The application queries.
• The relative frequency of query in application.
• Current indexes for your application.
• Which indexes most common query use.

21. • Use indexes to sort query result
• Ensure that your indexes fit entirely in RAM
• Consider Performance when Creating Indexes for Write-
heavy Applications

22. Review
• Understand your workload
– Profile your queries
– Use explain on the slow ones
• Create indexes for slow operations