mongodb-indexes

问题 If you have a double compound index { a : 1, b : 1}, it makes sense to me that the index won't be used if you query on b alone (i.e. you cannot "skip" a in your query). The index will however be used if you query on a alone. However, given a triple compound index { a : 1, b: 1, c: 1} my explain command is showing that the index is used when you query on a and c (i.e. you can "skip" b in your query). How can Mongo use an abc index on a query for ac , and how effective is the index in this case

问题 Is there a way to see a list of indices on a collection in mongodb in shell? i read through http://www.mongodb.org/display/DOCS/Indexes but i dont see anything 回答1: From the shell: db.test.getIndexes() For shell help you should try: help; db.help(); db.test.help(); 回答2: And if you want to get list of all indexes in your database: use "yourdbname" db.system.indexes.find() 回答3: If you want to list all indexes: db.getCollectionNames().forEach(function(collection) { indexes = db[collection]

问题 Is there a way to see a list of indices on a collection in mongodb in shell? i read through http://www.mongodb.org/display/DOCS/Indexes but i dont see anything 回答1: From the shell: db.test.getIndexes() For shell help you should try: help; db.help(); db.test.help(); 回答2: And if you want to get list of all indexes in your database: use "yourdbname" db.system.indexes.find() 回答3: If you want to list all indexes: db.getCollectionNames().forEach(function(collection) { indexes = db[collection]

问题 I'm not sure I understand sparse indexes correctly. I have a sparse unique index on fbId { "ns" : "mydb.users", "key" : { "fbId" : 1 }, "name" : "fbId_1", "unique" : true, "sparse" : true, "background" : false, "v" : 0 } And I was expecting that would allow me to insert records with null as the fbId, but that throws a duplicate key exception. It only allows me to insert if the fbId property is removed completely. Isn't a sparse index supposed to deal with that? 回答1: Sparse indexes do not

问题 Using the code: all_reviews = db_handle.find().sort('reviewDate', pymongo.ASCENDING) print all_reviews.count() print all_reviews[0] print all_reviews[2000000] The count prints 2043484 , and it prints all_reviews[0] . However when printing all_reviews[2000000] , I get the error: pymongo.errors.OperationFailure: database error: Runner error: Overflow sort stage buffered data usage of 33554495 bytes exceeds internal limit of 33554432 bytes How do I handle this? 回答1: You're running into the 32MB

问题 I have created a mongoose model that has an email field. I want it to be unique if a value is provided by a user but I want it to be empty is a user has not provided any value. I have found a good mongodb reference here: https://docs.mongodb.com/manual/core/index-partial/#partial-index-with-unique-constraints that could work but I don't know how to make it work on mongoose This is how the field looks like right now email: { type: String, index: true, unique: true } If I leave it the way it is

问题 Suppse I created index like following :- db.collection.createIndex( { propA: 1, propB: 1, propC:1 } ) and I query like following :- db.collection.find({propB:'x', propC: 'y', propA:'z'}) will mongo query engine use the index created above or not. Does sequence of key matters in usage of compound index while writing query? 回答1: The order of keys in a query doesn't matter: MongoDB is smart enough to look at all the queried properties and find a suitable index. However, the order of keys defined

问题 We're storing documents with a undefined structure. I mean, it has a basic structure ( id , user and creationTimestamp ), but is also there a Map<String, Object> values fields, where we are able to store whichever structure: public class Metadata { private String id; private String user; private Date creationTimestamp; private Map<String, Object> values; } Example: > db.metadata.find(); { "_id" : "Doc2Ref2Mdt1", "user" : "user1", "creationTimestamp" : ISODate("2018-09-24T12:20:56.958Z"),

问题 Question: I have a very large collection that is indexed by field ts : (timestamp) > db.events.ensureIndex({'ts': -1}) I want to get last 5 entries. What surprises me is that the query doesn't use the index and is thus very slow: > db.events.find().sort({'ts': -1, '_id': -1}).limit(5) However, sorting just by ts or the other field uses index as it should: > db.events.find().sort({'ts': -1}).limit(5) > db.events.find().sort({'_id': -1}).limit(5) Is this a bug in MongoDB, is this indeed a

问题 I'm currently trying to build an 2dsphere index, but the creation seems to fail. The document on which the index creation fails is valid geojson (according to geojsonlint). Also as far as I can see it obeys the MongoDB "Polygon" rules. I would appreciate any help, since I can't figure out why the index creation seems to fail. Thanks in advance! db.poly.ensureIndex( { loc: "2dsphere" } ) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 1, "ok" : 0, "errmsg" : "Can't extract geo