Compound index and field order in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a compound index in MongoDB?

A compound index is an index on multiple fields in a MongoDB document. It helps speed up queries that filter or sort by those fields together.

intermediate

Why does the order of fields matter in a compound index?

The order matters because MongoDB uses the index from left to right. Queries must match the prefix of the index fields to use it efficiently.

intermediate

Given a compound index on { age: 1, name: 1 }, which queries can use this index?

Queries filtering by age alone or by both age and name can use this index. Queries filtering only by name cannot use it efficiently.

intermediate

How does MongoDB use a compound index for sorting?

MongoDB can use a compound index to sort results if the sort order matches the index field order and direction.

advanced

What happens if you reverse the field order in a compound index?

Reversing the field order creates a different index. Queries optimized for the original order may not benefit from the reversed index.

Which of these is a valid compound index in MongoDB?

A { age }

B { age: 'ascending', name: 'descending' }

C { name: 1 }

D { age: 1, name: 1 }

Compound indexes specify multiple fields with sort order (1 for ascending, -1 for descending). Option D is correct.

If you have a compound index on { age: 1, name: 1 }, which query uses the index efficiently?

A { age: 25 }

B { name: 'Alice' }

C { name: 'Alice', age: 25 }

D { city: 'NY' }

Queries filtering on the first field (age) or both fields can use the index. Filtering only by name cannot.

What is the effect of field order in a compound index?

A It does not matter

B MongoDB uses fields from left to right for queries

C Only the last field is used

D Fields are used randomly

MongoDB uses the compound index fields in the order they are defined, from left to right.

Can a compound index be used to sort query results?

A Yes, if the sort matches the index field order

B No, indexes are only for filtering

C Only single-field indexes can sort

D Sorting is done after fetching all results

Compound indexes can support sorting if the sort order matches the index field order.

What happens if you create a compound index with reversed field order?

A MongoDB merges both indexes

B It creates the same index

C It creates a different index

D It causes an error

Reversing the field order creates a different compound index with different query optimization.

Explain what a compound index is and why the order of fields matters in MongoDB.

Think about how MongoDB uses indexes to speed up queries.

Describe how MongoDB uses a compound index to optimize queries and sorting.

Consider both filtering and sorting when using compound indexes.