MongoDB create Index. Indexing work

Preamble

When searching for documents in small collections, we won’t have any special problems. However, when collections contain millions of documents, and we need to sample them by a certain field, it may take some time to find the right data, which may be critical to our task. In this case, indexes may help us.

Indexes allow us to organize the data by a certain field, which later will speed up the search. For example, if we usually perform a search in our application or task by the name field, we can index the collection by this field:

db.users.createIndex({"name" : 1})

In this way, the createIndex method is used to set an index in the name field. MongoDB allows setting up to 64 indexes per collection.

Setting up the indexes

If we just define an index for the collection, for example db.users.createIndex({“name” : 1}), we can still add documents with the same value of the key name to the collection. However, if we need a document with the same key value to be added to the collection only once, we can set the flag unique:

db.users.createIndex({"name" : 1}, {"unique" : true})

Now, if we try to add two documents with the same name value to the collection, we get an error.

At the same time, there are some subtleties here. Thus, a document may not have the key name. In this case, the key name with null is automatically created for the document to be added. Therefore, when adding a second document, where the key name is not defined, an exception will be thrown, because the key name with the null value is already present in the collection.

It is also possible to specify a unique index for two fields at once:

db.users.createIndex({"name" : 1, "age" : 1}, {"unique" : true})

However, in this case, all added documents must have unique values for both fields.

Besides, there are some limitations here. For example, the value of the field, which is used for indexing, should not exceed 1024 bytes.

Index management

All database indexes are stored in the system.indexes collection. By accessing it, we can obtain all indexes and related information:

db.system.indexes.find()

We can also use the getIndexes method to display all index information for a particular collection:

db.users.getIndexes()

This command will return the output like the next one:

[
{
"v" : 1,
"key" : {
"_id" : 1
},
"ns" : "test.users",
"name" : "_id_"
},
{
"v" : 1,
"key" : {
"name" : 1
},
"ns" : "test.users",
"name" : "name_1"
}
]

As we can see, there are 2 indexes for the user’s collection (from the database test): id and name. The key field is used to find the maximum and minimum values, for various operations where this index must be applied. The name field is used as an identifier for administrative operations, for example, to delete an index:

db.users.dropIndex("name_1")

MongoDB Tutorial for Beginners 12 – MongoDB Indexing

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