$table->index(["paymentable_type", "paymentable_id"], 'paymentable_idx');

<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

return new class extends Migration
{
    private string $morphName = 'commentable';

    public function up()
    {
        Schema::table('posts', function (Blueprint $table) {
            $table->after('id', function (Blueprint $table) {
                $table->string("{$this->morphName}_type");
                $table->unsignedBigInteger("{$this->morphName}_id");
            });
            $table->index(["{$this->morphName}_type", "{$this->morphName}_id"], "idx_{$this->morphName}");
        });
    }

    /**
     * Reverse the migrations.
     */
    public function down(): void
    {
        Schema::table('posts', function (Blueprint $table) {
            $table->dropMorphs($this->morphName, "idx_{$this->morphName}");
        });
    }
};