Composite index requirements in Firebase - Step-by-Step Execution

Process Flow - Composite index requirements

Start Query

↓

Check Query Filters

↓

Are multiple fields filtered or sorted?

No→Use Single-field Index

Yes↓

Check Composite Index Exists

↓

If Exists

→Query Uses Composite Index

↓

If Not Exists

→Error: Create Composite Index

↓

Create Composite Index in Firebase Console

↓

Retry Query

↓

Query Succeeds Using Composite Index

When a query filters or sorts on multiple fields, Firebase requires a composite index. If missing, it prompts to create one before the query can succeed.

Execution Sample

Firebase

db.collection('books')
  .where('author', '==', 'Alice')
  .orderBy('year')
  .get()

This query filters books by author and sorts by year, requiring a composite index on 'author' and 'year'.

Process Table

Step	Query Part	Index Check	Action	Result
1	Filter on 'author'	Single-field index exists	Use single-field index	Continue
2	Order by 'year'	Needs composite index with 'author' and 'year'	Check composite index	Not found
3	Composite index missing	Error thrown	Prompt to create composite index	Query fails
4	Create composite index	Index created in Firebase Console	Retry query	Query succeeds

💡 Query stops at step 3 because composite index is missing for multiple fields filter and sort.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
Query Status	Not started	Filtering on author OK	Composite index missing error	Error shown to user	Query succeeds after index creation

Key Moments - 2 Insights

Why does the query fail even though single-field indexes exist?

Because the query filters and sorts on multiple fields, Firebase requires a composite index combining those fields (see execution_table step 3). Single-field indexes alone are not enough.

What should I do when Firebase says a composite index is missing?

You need to create the composite index in the Firebase Console as prompted (see execution_table step 4), then retry the query.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the query fail due to missing composite index?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Check the 'Result' column in execution_table row for step 3.

According to variable_tracker, what is the query status after step 4?

A Query fails with error

B Query succeeds after index creation

C Query not started

D Filtering on author OK

💡 Hint

Look at the last column 'After Step 4' in variable_tracker for 'Query Status'.

If the query only filtered on 'author' without ordering by 'year', what index would be needed?

A Single-field index on 'author'

B No index needed

C Composite index on 'author' and 'year'

D Composite index on 'year' only

💡 Hint

Refer to execution_table step 1 where filtering on 'author' uses single-field index.

Concept Snapshot

Composite index is needed when a query filters or sorts on multiple fields.
Firebase checks if such an index exists.
If missing, query fails with an error.
Create the composite index in Firebase Console.
Retry query to succeed.

Full Transcript

When you run a Firebase query that filters or sorts on more than one field, Firebase requires a composite index that covers those fields. The process starts by checking if single-field indexes suffice. If the query involves multiple fields, Firebase looks for a composite index. If it does not exist, the query fails and Firebase prompts you to create the composite index in the Firebase Console. After creating the index, retrying the query will succeed. This ensures queries are efficient and fast.