Indexing documents
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The document provides an overview of indexing in MongoDB, explaining how separate tree structures are used to improve search efficiency. It covers the creation of various types of indices, including automatic, compound, and unique indices, and discusses their impact on search queries and operations. Additionally, it addresses considerations regarding the performance trade-offs of indices and offers commands for monitoring index usage and sizes.
![More Exact Matching# index over sku….find({sku: {$in:[1234,5678])# index over ‘product.sku’….find({“product.sku”:1234})# a tricky query, would need index on ‘product’ instead….find({product: {sku:1234}}) { _id:1, product: {sku:1234} } # matches](https://image.slidesharecdn.com/indexingdocuments-100624110404-phpapp01/85/Indexing-documents-7-320.jpg)
![Array MatchingA field that contains an array will have one entry in the index per element in the array{ _id: “abcd”, x:[2,10]} will appear in all the following queries using an index over x….find({x:2})….find({x:10})….find({x:[2,10]})….find({x:{$gt:5}}) # because of 10](https://image.slidesharecdn.com/indexingdocuments-100624110404-phpapp01/85/Indexing-documents-13-320.jpg)
Indexing documents
- 1. Indexing Documents in MongoDBAlberto LernerSoftware Engineer – 10Genalerner@10gen.com
- 2. 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
- 3. Team Work MongoDB’s job: use an index, if possibleSearchCriteriausing indexscanning the collection
- 4. Your JobTo provide indices for important queriesImportant queries?Very frequently usedEspecially low response time required
- 5. 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})
- 6. 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>
- 7. More Exact Matching# index over sku….find({sku: {$in:[1234,5678])# index over ‘product.sku’….find({“product.sku”:1234})# a tricky query, would need index on ‘product’ instead….find({product: {sku:1234}}) { _id:1, product: {sku:1234} } # matches
- 8. Range CriteriaSearch criteria may return several resultsdb.<collection>.findOne({sku: {$gt:1234}}) db.<collection>.find({sku: {$gt:5678,$lt:5699}})
- 9. Range Criteria (cont)# index over sku….find({sku:/^12/}
- 10. Other Operations# index over sku….update({sku:1234},{$inc:{sold:1}}})….remove({sku:1234})
- 11. Index CoveringSometimes, all the needed information is in the index itself….count({color:blue}) # index over color….find({sku:1234},{color:1}) # index over sku, color
- 12. Missing fieldsAll documents have an entry on an indexA missing field is indexed as a NULL# matches all documents without sku# if index over sku is unique, there could be only one….find({sku:NULL}) # will be using a sku index, but not there yet….find({{sku:{$exits:true}})
- 13. 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:{$gt:5}}) # because of 10
- 14. Indexes and OrderingSort elimination is also accomplished though using indexes….find({sku:{$gt:56678}).sort({sku:1})….find().sort({sku:-1}) # can traverse backwards
- 15. Is It Using the Index?explain() tool allows you to see whether an index is being chosendb.<collection>.find({sku:{$gt:5}}).explain(){“cursor” : “BtreeCursor sku_1”,…}
- 16. HintingSometimes we may force or avoid the use of an indexUsually, it should not be necessary to intervene# forces use of index over sku….find{{sku:1, …}).hint({sku:1})# prevents any index to be used….find({sku:1,…}).hint({$natural:1})
- 17. 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”})
- 18. 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
- 19. 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)
- 20. 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()
- 21. Take awayThe picture to keep in mindSearchCriteria