Sparse indexes in MongoDB - Step-by-Step Execution

Concept Flow - Sparse indexes

Create Sparse Index

↓

Check Each Document

↓

Does Document Have Indexed Field?

No→Skip Document

Yes↓

Add Document to Index

↓

Index Ready for Queries

When creating a sparse index, MongoDB checks each document. It only adds documents that have the indexed field to the index. Documents missing the field are skipped.

Execution Sample

MongoDB

db.users.createIndex({email: 1}, {sparse: true})

Creates a sparse index on the 'email' field in the 'users' collection, indexing only documents that have the 'email' field.

Execution Table

Step	Document _id	Has 'email' Field?	Action	Index State
1	1	Yes	Add to index	Index contains _id:1
2	2	No	Skip document	Index contains _id:1
3	3	Yes	Add to index	Index contains _id:1, _id:3
4	4	No	Skip document	Index contains _id:1, _id:3
5	5	Yes	Add to index	Index contains _id:1, _id:3, _id:5
6	End	-	All documents checked	Final index: _id:1, _id:3, _id:5

💡 All documents processed; only those with 'email' field included in index.

Variable Tracker

Variable	Start	After Doc 1	After Doc 2	After Doc 3	After Doc 4	After Doc 5	Final
Index Contents	empty	[_id:1]	[_id:1]	[_id:1, _id:3]	[_id:1, _id:3]	[_id:1, _id:3, _id:5]	[_id:1, _id:3, _id:5]

Key Moments - 2 Insights

Why are some documents not included in the sparse index?

Documents without the indexed field are skipped during index creation, as shown in steps 2 and 4 of the execution_table where documents without 'email' are not added.

Does a sparse index include documents with the indexed field set to null?

Yes, documents with the field present, even if null, are included. Only documents missing the field entirely are skipped.

Visual Quiz - 3 Questions

Look at the execution_table, which document is the first to be skipped from the index?

A Document with _id:2

B Document with _id:1

C Document with _id:3

D Document with _id:5

💡 Hint

Check the 'Has email Field?' and 'Action' columns in the execution_table for step 2.

At the end of the process, how many documents are included in the sparse index?

A 2

B 4

C 3

D 5

💡 Hint

Look at the 'Index State' column in the final row of the execution_table.

If a document has the 'email' field set to null, will it be included in the sparse index?

A No, because null is treated as missing

B Yes, because the field exists

C No, because sparse indexes exclude null values

D Only if the index is unique

💡 Hint

Refer to the key_moments section explaining inclusion of documents with null fields.

Concept Snapshot

Sparse indexes in MongoDB index only documents that have the indexed field.
Documents missing the field are skipped.
Useful to save space and speed up queries on optional fields.
Syntax: createIndex({field: 1}, {sparse: true})
Queries on sparse indexes ignore documents without the field.

Full Transcript

Sparse indexes in MongoDB work by including only those documents that have the indexed field. When you create a sparse index, MongoDB checks each document in the collection. If the document has the field you want to index, it adds it to the index. If the document does not have that field, it skips it. This means the index is smaller and faster for queries that involve that field. For example, if you create a sparse index on the 'email' field, only documents with an 'email' will be in the index. Documents without 'email' are not indexed. This behavior is shown step-by-step in the execution table, where documents with and without the 'email' field are processed. Sparse indexes also include documents where the field exists but is null. This is important to remember. Sparse indexes are useful when the field is optional and not present in every document.