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Use the ensureIndex function to create an index of the collection.

For the collection, each index only needs to be created once, and repeated creation has no effect.

> show collections system.indexes users

The index metadata of all databases is stored in the collection of system.indexs

Use the getIndexes function to view the index information on a given collection:

> db.users.getIndexes () [{"v": 1, "key": {"_ id": 1}, "name": "_ id_", "ns": "blog.users"}, {"v": 1 "key": {"username": 1}, "name": "username_1", "ns": "blog.users"}, {"v": 1, "key": {"age": 1 "username": 1}, "name": "age_1_username_1", "ns": "blog.users"}] >

It can be adapted to the dropIndex function to delete the specified index:

> db.users.dropIndex ({"username": 1}) {"nIndexesWas": 3, "ok": 1} > db.users.getIndexes () [{"v": 1, "key": {"_ id": 1}, "name": "_ id_" "ns": "blog.users"}, {"v": 1, "key": {"age": 1, "username": 1}, "name": "age_1_username_1" "ns": "blog.users"}] >

In the above example, you can see that the default format of index identification is keyname1_dir1_keyname2_dir. .keynameN _ DirN

You can also specify an identity name:

> db.users.ensureIndex ({"username": 1}, {"name": "firstname"}) {"createdCollectionAutomatically": false, "numIndexesBefore": 2, "numIndexesAfter": 3, "ok": 1} > db.users.getIndexes () [{"v": 1 "key": {"_ id": 1}, "name": "_ id_", "ns": "blog.users"}, {"v": 1, "key": {"age": 1 "username": 1}, "name": "age_1_username_1", "ns": "blog.users"}, {"v": 1, "key": {"username": 1} "name": "firstname", "ns": "blog.users"}] >

To modify an index: delete index-> create index

Deleting an index can specify the index identity directly, for example:

> db.users.dropIndex ("firstname") {"nIndexesWas": 3, "ok": 1} > db.users.getIndexes () [{"v": 1, "key": {"_ id": 1}, "name": "_ id_" "ns": "blog.users"}, {"v": 1, "key": {"age": 1, "username": 1}, "name": "age_1_username_1" "ns": "blog.users"}] >

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