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What is an index, how do they work and when are they used... this talk will show you the in's and out's on indexing in MongoDB.
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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
Indexes Maintain Order
Index on a: ascending
{a: 0, b: 9}{a: 2, b: 0}
{a: 7, b: 1}
{a: 3, b: 2}
{a: 3, b: 5}{a: 3, b: 7}
{a: 9, b: 1}
Indexes Maintain Order
Index on a: ascending, b: descending
{a: 0, b: 9}{a: 2, b: 0}
{a: 7, b: 1}
{a: 3, b: 7}
{a: 3, b: 2}{a: 3, b: 5}
{a: 9, b: 1}
B-tree StructureIndex on a: ascending
{...} {...} {...} {...} {...} {...} {...} {...} {...} {...} {...}
[-‐∞, 5)[5, 10)
[10, ∞)
[5, 7) [7, 9) [9, 10)[10, ∞) buckets[-‐∞, 5) buckets
Query for {a: 7}
{...} {...} {...} {...} {...} {...} {...} {...} {...} {...} {...}
[-‐∞, 5)[5, 10)
[10, ∞)
[5, 7) [7, 9) [9, 10)[10, ∞) buckets[-‐∞, 5) buckets
With Index
Without index -‐ Scan
Creating Indexes
db.posts.ensureIndex({“name”: 1})
1 = ascending-‐1 = descending
An index on _id is automatic.
For more use ensureIndex:
Compound Indexes
db.posts.ensureIndex({name: 1, date: -‐1})
Unique Indexes
db.posts.ensureIndex({title: 1}, {unique: true})
Background Index Creation
db.posts.ensureIndex(..., {background: true})
Indexing 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 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}))
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.coll.find({a: 0})
db.coll.find({a: {$in: [0, 2]}})
db.coll.find({a: {$gt: 5}})
db.coll.count({a: 0})db.coll.find().sort({a: -‐1})db.coll.find({a: 0}, {a:1, _id:0})
db.coll.find({b: 0}).sort({a: -‐1})
Partially:
When isn’t an Index Used?
Index on {a: 1, b: -‐1}
db.collection.find({b: 0})
Picking an a Index
find({x: 10, y: “foo”})
scan
index on x
index on y remember
terminate
When are Indexes Needed?
Frequently used queriesLow response time
Indexes Take Up Space
db.collection.totalIndexSize()
Indexes Slow Down Writes
Does my query use an Index?db.collection.find(query).explain();
Explain - Scan all documentsdb.coll.find({title:”My blog”}).explain();
{ "cursor" : "BasicCursor", "indexBounds" : [ ], "nscanned" : 57594, "nscannedObjects" : 57594, "n" : 3, "millis" : 108}
Explain - Index used
{ "cursor" : "BtreeCursor x_1", "indexBounds" : [ ], "nscanned" : 123, "nscannedObjects" : 123, "n" : 10, "millis" : 4}
db.coll.ensureIndex({title:1});db.coll.find({title:”My blog”}).explain();
Explain - Covered Index used
{ "cursor" : "BtreeCursor x_1", "indexBounds" : [ ], "nscanned" : 123, "nscannedObjects" : 123, "n" : 10, "millis" : 4, "indexOnly" : true}
db.coll.ensureIndex({title:1});db.coll.find({title:”My blog”}, {title:1, _id:0}).explain();
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