SELECT 
  product_id, 
  parent_product_id 
FROM 
  cscart_product_variation_group_products 
WHERE 
  parent_product_id = 18508

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00311277,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_variation_group_products",
          "access_type": "ref",
          "possible_keys": ["idx_parent_product_id"],
          "key": "idx_parent_product_id",
          "key_length": "3",
          "used_key_parts": ["parent_product_id"],
          "ref": ["const"],
          "loops": 1,
          "rows": 10,
          "cost": 0.00311277,
          "filtered": 100,
          "using_index": true
        }
      }
    ]
  }
}