SELECT 
  gp.*, 
  p.company_id 
FROM 
  cscart_product_variation_group_products AS gp 
  LEFT JOIN cscart_products AS p ON gp.product_id = p.product_id 
WHERE 
  gp.group_id IN (2004)

Query time 0.00073

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "43.20"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": [
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 18,
          "rows_produced_per_join": 18,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "18.00",
            "eval_cost": "3.60",
            "prefix_cost": "21.60",
            "data_read_per_join": "288"
          },
          "used_columns": [
            "product_id",
            "parent_product_id",
            "group_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "p",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "webmarco.gp.product_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 18,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "18.00",
            "eval_cost": "3.60",
            "prefix_cost": "43.20",
            "data_read_per_join": "140K"
          },
          "used_columns": [
            "product_id",
            "company_id"
          ]
        }
      }
    ]
  }
}