Indexing

Indexing Mike Dirolf -‐ @mdirolf -‐ 10gen, Inc. http://www.mongodb.org

What’s Easy About MongoDB Indexing? It’s almost the same as in your RDBMS

What’s Hard About MongoDB Indexing? It’s almost the same as in your RDBMS

What is an Index? Magic scaling sauce?

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

Indexes Maintain Order Index on {a: 1} {a: 0, b: 9} {a: 2, b: 0} {a: 3, b: 2} {a: 3, b: 7} {a: 3, b: 5} {a: 7, b: 1} {a: 9, b: 1}

Indexes Maintain Order Index on {a: 1, b: -‐1} {a: 0, b: 9} {a: 2, b: 0} {a: 3, b: 7} {a: 3, b: 5} {a: 3, b: 2} {a: 7, b: 1} {a: 9, b: 1}

B-tree Structure Index on {a: 1} [-∞, 5) [5, 10) [10, ∞) [-∞, 5) buckets [5, 7) [7, 9) [9, 10) [10, ∞) buckets {...} {...} {...} {...} {...} {...} {...} {...} {...} {...} {...}

Query for {a: 7} Index [-∞, 5) [5, 10) [10, ∞) [-∞, 5) buckets [5, 7) [7, 9) [9, 10) [10, ∞) buckets {...} {...} {...} {...} {...} {...} {...} {...} {...} {...} {...} Scan

Creating Indexes An index on _id is automatic. For more, ensureIndex: db.posts.ensureIndex({“name”: 1})

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

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

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

Indexing Embedded Documents db.posts.ensureIndex({“comments.author”: 1})

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

Geospatial db.posts.ensureIndex({“location”: “2d”})

Listing Indexes db.posts.getIndexes()

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

When is an Index Used? Index on {a: 1} db.collection.find({a: 0}) db.collection.find({a: {$in: [0, 2]}}) db.collection.find({a: {$gt: 5}}) db.collection.count({a: 0}) db.collection.find().sort({a: -‐1}) Partially: db.collection.find({b: 0}).sort({a: -‐1})

When isn’t an Index Used? Index on {a: 1, b: -‐1} db.collection.find({b: 0}) As a rule: try imagining how the sorted representation could help the server with your query.

Picking an Index find({x: 10, y: “foo”}) scan terminate index on x index on y remember

When are Indexes Needed? Frequently used queries Low response time

Indexes Take Up Space db.collection.totalIndexSize()

Indexes Slow Down Writes

Explain db.collection.find(query).explain(); { "cursor" : "BasicCursor", "indexBounds" : [ ], "nscanned" : 57594, "nscannedObjects" : 57594, "n" : 3 , "millis" : 108 }

Explain { "cursor" : "BtreeCursor x_1", "indexBounds" : [ ], "nscanned" : 123, "nscannedObjects" : 123, "n" : 10 , "millis" : 4 }

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Indexing

by Mike Dirolf on Sep 21, 2010

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Indexing — Presentation Transcript