Unexpected Index Omission with constrained() and Composite unique()

Hello everyone,

I'm trying to get a definitive clarification on the index creation behavior of ->constrained() when used in a table that also has a composite unique index. I've encountered a scenario where an expected index is not being created, and I'd like to understand if this is intended behavior.

Context:

• Laravel Version: 12.x
• PostgreSQL Version: 17.x

The Scenario

Let's say I have three migrations to create authors, categories, and a books table.

1. authors table:

Schema::create('authors', function (Blueprint $table) {
    $table->id();
    $table->string('name');
    $table->timestamps();
});

2. categories table:

Schema::create('categories', function (Blueprint $table) {
    $table->id();
    $table->string('name')->unique();
    $table->timestamps();
});

3. books table: This is where the unexpected behavior occurs. The table has two foreign keys and a composite unique index.

Schema::create('books', function (Blueprint $table) {
    $table->id();
    $table->foreignId('author_id')->constrained();
    $table->foreignId('category_id')->constrained();
    $table->string('title');
    $table->timestamps();

    // A book title must be unique for a given author.
    $table->unique(['author_id', 'title']);
});

The Expectation

Based on Laravel's conventions, I expected the books table to have three performance indexes:

1. A primary key index on id.
2. A unique composite index on (author_id, title).
3. A simple B-Tree index on category_id (created by the ->constrained() method).

The simple index on author_id would be correctly omitted as redundant because author_id is the leading column in the composite unique index.

The Actual Result

However, after running the migration and inspecting the database directly with the following query:

SELECT indexname, indexdef
FROM pg_indexes
WHERE tablename = 'books';

The result only shows two indexes:

indexname	indexdef
books_pkey	CREATE UNIQUE INDEX books_pkey ON public.books USING btree (id)
books_author_id_title_unique	CREATE UNIQUE INDEX books_author_id_title_unique ON public.books USING btree (author_id, title)

The expected index on category_id is missing.

The Question

Is this omission of the category_id index the intended behavior of Laravel's schema builder?

It seems that the presence of the unique(['author_id', 'title']) composite index is preventing the creation of other expected indexes from ->constrained(). This could lead to silent performance issues on queries filtering by category_id.

Any clarification on whether this is a bug or an undocumented optimization feature would be greatly appreciated!

Thanks!