Compound index where one field can be null MongoDB - mongodb

Question

How can I create compound index in mongo where one of the fields maybe not present or be null?
For example in below documents if I create a compound index name+age. How can I still achieve this with age being not present or null in some documents?
{
name: "Anurag",
age: "21",
},
{
name: "Nitin",
},

Answer 1

You can create partial Index as follow:
db.contacts.createIndex(
{ name: 1 },
{ partialFilterExpression: { age: { $exists: true } } }
)
Explained:
As per the documentation partial indexes only index the documents in a collection that meet a specified filter expression. By indexing a subset of the documents in a collection, partial indexes have lower storage requirements and reduced performance costs for index creation and maintenance. In this particular case imagine your collection have 100k documents , but only 5 documents have the "age" field existing , in this case the partial index will include only those 5 fields in the index optimizing the index storage space and providing better performance.
For the query optimizer to choose this partial index, the query predicate must include a condition on the name field as well as a non-null match on the age field.
Following example queries will be able to use the index:
db.contacts.find({name:"John"})
db.contacts.find({name:"John",age:{$gt:20}})
db.contacts.find({name:"John",age:30})
Following example query is a "covered query" based on this index:
db.contacts.find({name:"John",age:30},{_id:0,name:1,age:1})
( this query will be highly efficient since it return the data directly from the index )
Following example queries will not be able to use the index:
db.contacts.find({name:"John",age:{$exists:false}})
db.contacts.find({name:"John",age:null})
db.contacts.find({age:20})
Please, note you need to perform some analysis on if you need to search on the age field together with the name , since name field has a very good selectivity and this index will not be used in case you search only by age , maybe a good option is to create additional sparse/partial index only on the age field so you could fetch a list with contacts by certain age if this a possible search use case.