Indexes for MongoDB learning and sorting

The index information is stored in system. indexes.

Create an index:
DB. c1.ensureindex ({age: 1 },{ Background: true })
1: ascending-1: Descending
Background: whether to create an index in the background

Query the index of a table
DB. c1.getindexes () or db. system. Indexes. Find ()

Document index: the index will be created on fields of the embedded document type.
DB. Factories. insert ({name: "Baidu", ADDR: {City: "Beijing", state: "BJ "}})
DB. Factories. ensureindex ({ADDR: 1 })
DB. Factories. Find ({ADDR: {City: "Beijing", state: "BJ"}) -- Take the index
DB. Factories. Find ({ADDR: {state: "BJ", city: "Beijing"}) -- no index is taken and no data can be queried, because the order is inconsistent.

Combined index:
DB. Factories. ensureindex ({"ADDR. City": 1, "ADDR. State": 1 })
DB. Factories. Find ({"ADDR. City": "Beijing", "ADDR. State": "BJ"}) -- Take the index
DB. Factories. Find ({"ADDR. City": "Beijing"}) -- Take the index
DB. Factories. Find ({"ADDR. State": "BJ", "ADDR. City": "Beijing"}) -- do not go to the index, because the order is inconsistent

Unique index: ensureindex added to unique: True
DB. t4.ensureindex ({firstname: 1, lastname: 1 },{ unique: true })

Forcible indexing: using the hint command does not necessarily speed up indexing for all operations.
DB. t5.insert ({name: "liangzhangping", Age: 28}) -- add data for testing
DB. t5.ensureindex ({Name: 1, age: 1}) -- create an index
DB. t5.find ({age: {$ LT: 30}). Explain () -- check the execution plan to see if the index has been performed.

DB. t5.find ({age: {$ LT: 30}). Hint ({Name: 1, age: 1}). Explain ()

Delete an index and run the dropindex command.
1) Delete All indexes in the table, such as deleting all indexes in Table T3.
DB. t3.dropindexes ()
2) delete an index in a table. For example, delete the index created on the firstname field in Table T4:
DB. t4.dropindex ({firstname: 1 })

Re-indexing: Use the reindex command, for example, to re-create all indexes on table T3:
DB. t3.reindex ()

Optimizer: profiler, used to analyze the operation time for debugging and optimization. Information is saved in system. profile, and system. profile is a capped collection.

Start profiler:
1) Add "-profile = level" to start MongoDB"
/APP/Mongo/MongoDB/bin/mongod -- profile = 2
2) execute the DB. setprofilinglevel (level) command on the client, for example:
Primary> dB. setprofilinglevel (1)
Obtain the current level: DB. getprofilinglevel ()
Level:
0: Disabled
1: Record slow commands
2: record all commands

DB. setprofilinglevel (): records operations that exceed 10 ms