Indexing & query optimization

Indexing & Query Optimization
Jared Rosoff ( [email_address] )

Overview
Indexing 101
Profiling your queries
Creating Indexes
Managing Index

Indexing 101

Ack! My queries are slow! http://michaelprescott.typepad.com/.a/6a00d83451574c69e20134858a87a2970c-800wi

Index FTW! http://musformation.com/pics/excited.jpg

Why did that happen? Magic More Magic

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

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

O(n) vs. O(log n) Table scan Index Number of records Number of comparisons

Analyzing Query Performance

Using the Profiler
db.setProfilingLevel( level )
0 == off
1 == slow operations (>100ms)
2 == all operations

Profiler Output
db.system.profile.find ({millis:{$gt:5}});
{
"ts" : "Thu Jan 29 2009 15:21:27 GMT-0500 (EST)" ,
"info" : "query test.foo ntoreturn:0 exception bytes:53" ,
"millis" : 88
}

Use explain
query = db.coll.find({title:”My blog”})
query.explain();
{ "cursor" : " BasicCursor ", "indexBounds" : [ ], "nscanned" : 57594, "nscannedObjects" : 57594, "n" : 3, "millis" : 108 }

Creating Indexes

Index a field
db.posts.ensureIndex( { ‘name’: 1 })
1 = ascending
-1 = descending

Compound indexes
db.posts.ensureIndex({name: 1, date: -1})

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

Embedded documents
db.posts.save({
title: “My First blog”,
comments: [
{author: “James”, ts : new Date()} ]
});
db.posts.ensureIndex({“ comments.author ” : 1})

Multikeys
{ “ tags ” : [ “ mongodb ” , “ cool ” ], ...}
db.posts.ensureIndex({ “ tags ” : 1})

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

Sparse Indexes

Geospatial Indexes

Managing Indexes

Listing indexes db.posts.getIndexes()

Dropping indexes db.posts.dropIndex({“tags”: 1})

Background building db.posts.ensureIndex(..., {background: true})

Query Planning

When isn’t an index used?

Picking an Index scan Index on x Index on y Remember Terminate

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

Indexing & query optimization

by Jared Rosoff on Feb 08, 2011

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Indexing & query optimization — Presentation Transcript