Indexing documents

Indexing Documents in MongoDBAlberto LernerSoftware Engineer – 10Genalerner@10gen.com

Indexing BasicsMongoDB can use separate tree structures to index a collectionWhen processing a search criteria, MongoDB will try to avoid going through a collection, taking advantage of existing indices

Team Work MongoDB’s job: use an index, if possibleSearchCriteriausing indexscanning the collection

Your JobTo provide indices for important queriesImportant queries?Very frequently usedEspecially low response time required

Creating an IndexYou have an automatic one over _idOthers can be created with ‘ensureIndex’# index over attribute ‘name’db.<collection>.ensureIndex({name:1})# compound keys, ascending/descendingdb.<collection>.ensureIndex({name:1, date:-1 })# unique keysdb.<collection>.ensureIndex({sku:1}, {unique:true})# building in backgrounddb.<collection>.ensureIndex( …, {background:true})

Simple Search CriteriaSearch criteria is the index key or a prefix thereofdb.<collection>.find({sku:1234}) # index over skudb.<collection>.find({sku:1234}) # index over sku, <xxx>

More Exact Matching# index over sku….find({sku: {$in:[1234,5678])# index over ‘product.sku’….find({“product.sku”:1234})# a tricky query, would need index on ‘product’ instead….find({product: {sku:1234}}) { _id:1, product: {sku:1234} } # matches

Range CriteriaSearch criteria may return several resultsdb.<collection>.findOne({sku: {$gt:1234}}) db.<collection>.find({sku: {$gt:5678,$lt:5699}})

Range Criteria (cont)# index over sku….find({sku:/^12/}

Other Operations# index over sku….update({sku:1234},{$inc:{sold:1}}})….remove({sku:1234})

Index CoveringSometimes, all the needed information is in the index itself….count({color:blue}) # index over color….find({sku:1234},{color:1}) # index over sku, color

Missing fieldsAll documents have an entry on an indexA missing field is indexed as a NULL# matches all documents without sku# if index over sku is unique, there could be only one….find({sku:NULL}) # will be using a sku index, but not there yet….find({{sku:{$exits:true}})

Array MatchingA field that contains an array will have one entry in the index per element in the array{ _id: “abcd”, x:[2,10]} will appear in all the following queries using an index over x….find({x:2})….find({x:10})….find({x:[2,10]})….find({x:{$gt:5}}) # because of 10

Indexes and OrderingSort elimination is also accomplished though using indexes….find({sku:{$gt:56678}).sort({sku:1})….find().sort({sku:-1}) # can traverse backwards

Is It Using the Index?explain() tool allows you to see whether an index is being chosendb.<collection>.find({sku:{$gt:5}}).explain(){“cursor” : “BtreeCursor sku_1”,…}

HintingSometimes we may force or avoid the use of an indexUsually, it should not be necessary to intervene# forces use of index over sku….find{{sku:1, …}).hint({sku:1})# prevents any index to be used….find({sku:1,…}).hint({$natural:1})

When Indexes Don’t Help# negation….find({sku:{$ne:9876}})# index helps to filter string sku’s, though….find({sku:/88/}) # generic regex# $where may contain very expressive searches# we don’t even try….find({$where:”this.sku==1234”})

Many indices?Evaluating search criteria currently uses just one index, even if more than one would be possibleThe choice is based on previous executions; if an index “worked well” for a query before, it’ll likely be againException: $or can use more than one index

So When to Index?There’s a trade off between search criteria efficiency and insertion/update/deletion of keysAlso, there is (a quite high) limit on number of indexes per collection (that we keep bumping up)

Indexes ResourcesIndexes are memory mapped as well; Be mindful of number of indexes and choice of keys# In ‘indexSizes’, individual indexes in collectiondb.<collection>.stats()# All indexes in collectiondb.<collection>.TotalIndexSize()

Take awayThe picture to keep in mindSearchCriteria

Indexing documents

More Related Content

What's hot

Similar to Indexing documents

More from MongoDB

Recently uploaded

Indexing documents