/swiftdata

SwiftData Indexing

Indexes speed up queries on frequently filtered or sorted properties.

Docs: Index macro

#Index (iOS 18+)

Single-Property Index

@Model
class Trip {
    @Attribute(.unique) var id: UUID
    var destination: String
    var startDate: Date
    var isFavorite: Bool

    init(destination: String, startDate: Date) {
        self.id = UUID()
        self.destination = destination
        self.startDate = startDate
        self.isFavorite = false
    }
}

Use #Index on the model to declare indexes:

@Model
class Trip {
    #Index<Trip>([\.destination], [\.startDate])
    // ...
}

This creates two single-column indexes: one on destination and one on startDate.

Compound Index

@Model
class Trip {
    #Index<Trip>([\.isFavorite, \.startDate])
    // ...
}

A compound index on (isFavorite, startDate) accelerates queries that filter on isFavorite and sort by startDate.

When to Index

Index when:

• A property appears in #Predicate filters on large collections (1000+ rows)
• A property is used in SortDescriptor on large collections

Don't index when:

• The collection is small (< few hundred rows)
• The property is rarely queried
• The property changes very frequently (index maintenance cost)
• The property has very low cardinality (e.g., a Boolean on a small table)
• Another schema constraint or existing index already covers the query pattern, unless profiling shows a separate index helps

Index and Migration

Treat adding or removing an index as a schema change. Test it on representative stores and keep a VersionedSchema / SchemaMigrationPlan ready for any release where the aggregate schema changes exceed SwiftData's automatic migration support.

Verifying Index Usage

Use Instruments > Core Data template to verify that queries use indexes. Look for "Full Table Scan" warnings on frequently executed fetches — these indicate a missing index.