what are idx_<pattern> indexes in tables and how they are generated or can be checked automatically?

[gab-chromatix]

I'm using Laravel (framework) version 10.48.8 and I was wondering when I try to get the table indexes using Schema::getIndexListing($table) it shows additional indexes that are autogenerated (idx_) but I didn't set it up in my migration script.

I'm executing this in my Unit test with sqlite as database.

Example migration script:

Schema::create('contacts', function (Blueprint $table) {
                $table->id();
                $table->foreignIdFor(User::class)->nullable()->constrained()->cascadeOnDelete()->nullOnDelete();
                $table->string('pageUrl')->nullable();
                $table->string('firstname');
                $table->string('lastname');
                $table->string('email')->nullable();
                $table->string('phone')->nullable();
                $table->text('message')->nullable();
                $table->foreignIdFor(Campaign::class)->nullable()->constrained()->cascadeOnDelete()->nullOnDelete();
                $table->foreignIdFor(Popup::class)->nullable()->constrained()->cascadeOnDelete()->nullOnDelete();

                $table->foreignId('test_id')->nullable()
                    ->references('id')->on('campaign_popups')
                    ->cascadeOnDelete()->nullOnDelete();

                $table->foreignIdFor(CampaignPopup::class)->nullable()->constrained()->cascadeOnDelete()->nullOnDelete();

                $table->text('dropdown')->nullable();
                $table->enum('type', ['contact', 'enquiry'])->default('contact');
                $table->timestamps();
                $table->softDeletes();
            });

Output for Schema::getIndexListing('contacts'):

array:6 [
  0 => array:5 [
    "name" => "idx_334015731e5d0459"
    "columns" => array:1 [
      0 => "test_id"
    ]
    "type" => null
    "unique" => false
    "primary" => false
  ]
  1 => array:5 [
    "name" => "idx_334015734ed37a4a"
    "columns" => array:1 [
      0 => "campaign_popup_id"
    ]
    "type" => null
    "unique" => false
    "primary" => false
  ]
  2 => array:5 [
    "name" => "idx_334015737ca3ac35"
    "columns" => array:1 [
      0 => "popup_id"
    ]
    "type" => null
    "unique" => false
    "primary" => false
  ]
  3 => array:5 [
    "name" => "idx_33401573a76ed395"
    "columns" => array:1 [
      0 => "user_id"
    ]
    "type" => null
    "unique" => false
    "primary" => false
  ]
  4 => array:5 [
    "name" => "idx_33401573f639f774"
    "columns" => array:1 [
      0 => "campaign_id"
    ]
    "type" => null
    "unique" => false
    "primary" => false
  ]
  5 => array:5 [
    "name" => "primary"
    "columns" => array:1 [
      0 => "id"
    ]
    "type" => null
    "unique" => true
    "primary" => true
  ]
] 

[shaedrich]

They come from foreignIdFor() (foreign key constraint).