The index of MongoDB

　　First, the concept of the index

I believe that everyone is not unfamiliar with the index, the index of the database is similar to the directory index of the book, there is an index, reading when you do not have to go through the whole book, you can directly jump to the target content according to the number of pages, improve reading and query efficiency. Database index is the same, it is used to improve the query speed, indexed, MongoDB query can be found in the index after the entry, directly to the target collection location.

　　Second, the default index

　　When retrieving the system index, it can be found that MongoDB defaults to a default "_id" index for each collection, which is used as the index to reference when retrieving. That is, our usual find operation is by default based on the "_id" index to find.

> Db.system.indexes.find (); {"V": 1, "key": {"_id": 1}, "ns": "Kaiye.c1", "name": "_id_"} {"V": 1, "key": {"_id": 1}, "ns": "Kaiye.c2", "name": "_id_"} {"V": 1, "key": {"_id": 1}, "ns": "kaiye.c3", "name": "_id_"}

Third, why to build a custom index

　　　　First add 1 million data to a set of C4, then retrieve the age=100 data, and call the explain function to calculate the information in the retrieval process, found that in the absence of sorting, no custom index, without querying a data, need to scan 1 million data, And just a simple query takes 546 milliseconds, which is unacceptable in a large data-volume business system.

for (Var i=0;i<1000000;i++) {
Db.c4.insert ({name: "Diao", age:i})

}

　　　　

　　Iv. Comparison of Custom indexes

　　　　First, the name field of the C4 collection is indexed, the keyword is ensureindex, the syntax is Db.c4.ensureIndex ({age:1}), and then the query age=100 operation is performed, retrieving information such as

At this point, you can find that after indexing the age field, a simple query is made, and the number of bars traversed is only one, and it takes almost 0, and the effect is generally visible compared to index-less retrieval.

　　　　

Iv. Other operations on the index

(1) Information about the query index:db. Collection. Stats ();

(2) Delete index: db. Collection. dropindex ({indexed field: 1})

　　　　　　　Deleting a collection also deletes all the indexes in the collection

　　　　 (3) Create a unique index: A unique index means that when a field is set to a unique index, the field in the collection, the same value is not allowed to exist, that is, the value is unique, such as the above case, and then insert a document AGE=100, there will be an error message.

Syntax: DB. Collection. Ensureindex ({age:1},{unique:true})

The index of MongoDB