Example
See the below transactions collection.
> db.transactions.insert({ cr_dr : "D", amount : 100, fee : 2});
> db.transactions.insert({ cr_dr : "C", amount : 100, fee : 2});
> db.transactions.insert({ cr_dr : "C", amount : 10, fee : 2});
> db.transactions.insert({ cr_dr : "D", amount : 100, fee : 4});
> db.transactions.insert({ cr_dr : "D", amount : 10, fee : 2});
> db.transactions.insert({ cr_dr : "C", amount : 10, fee : 4});
> db.transactions.insert({ cr_dr : "D", amount : 100, fee : 2});
getIndexes() functions will show all the indices available for a collection.
db.transactions.getIndexes();
Let see the output of above statement.
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "documentation_db.transactions"
}
]
There is already one index for transaction collection. This is because MongoDB creates a unique index on the _id field during the creation of a collection. The _id index prevents clients from inserting two documents with the same value for the _id field. You cannot drop this index on the _id field.
Now let's add an index for cr_dr field;
db.transactions.createIndex({ cr_dr : 1 });
The result of the index execution is as follows.
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
The createdCollectionAutomatically indicates if the operation created a collection. If a collection does not exist, MongoDB creates the collection as part of the indexing operation.
Let run db.transactions.getIndexes(); again.
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "documentation_db.transactions"
},
{
"v" : 1,
"key" : {
"cr_dr" : 1
},
"name" : "cr_dr_1",
"ns" : "documentation_db.transactions"
}
]
Now you see transactions collection have two indices. Default _id index and cr_dr_1 which we created. The name is assigned by MongoDB. You can set your own name like below.
db.transactions.createIndex({ cr_dr : -1 },{name : "index on cr_dr desc"})
Now db.transactions.getIndexes(); will give you three indices.
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "documentation_db.transactions"
},
{
"v" : 1,
"key" : {
"cr_dr" : 1
},
"name" : "cr_dr_1",
"ns" : "documentation_db.transactions"
},
{
"v" : 1,
"key" : {
"cr_dr" : -1
},
"name" : "index on cr_dr desc",
"ns" : "documentation_db.transactions"
}
]
While creating index { cr_dr : -1 } 1 means index will be in ascending order and -1 for descending order.
2.4
Hashed indexes
Indexes can be defined also as hashed. This is more performant on equality queries, but is not efficient for range queries; however you can define both hashed and ascending/descending indexes on the same field.
> db.transactions.createIndex({ cr_dr : "hashed" });
> db.transactions.getIndexes(
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "documentation_db.transactions"
},
{
"v" : 1,
"key" : {
"cr_dr" : "hashed"
},
"name" : "cr_dr_hashed",
"ns" : "documentation_db.transactions"
}
]
