Part of Medium article: https://medium.com/hoursofoperation/add-indexes-to-your-fluent-sql-databases-in-vapor-784f6b29a83f

fluent-index-migration.swift

import Fluent
import SQLKit

struct CreateVersionsCreatedAtIndex: AsyncMigration {
    func prepare(on database: Database) async throws {
        try await (database as! SQLDatabase)
            .create(index: "versions_created_at_index") // Index name
            .on("versions")                             // Table name
            .column("created_at")
            // You can also add more columns here if you want a multi-column index...
            .run()
    }

    func revert(on database: Database) async throws {
        try await (database as! SQLDatabase)
            .drop(index: "versions_created_at_index")
            .run()
    }
}