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 (1063)

Query time 0.00052

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "34.65"
    },
    "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": 59,
          "rows_produced_per_join": 59,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "7.24",
            "eval_cost": "5.90",
            "prefix_cost": "13.14",
            "data_read_per_join": "944"
          },
          "used_columns": [
            "product_id",
            "parent_product_id",
            "group_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "p",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY",
            "idx_products_inventory"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "cscart_db.gp.product_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 59,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "15.61",
            "eval_cost": "5.90",
            "prefix_cost": "34.65",
            "data_read_per_join": "307K"
          },
          "used_columns": [
            "product_id",
            "company_id"
          ]
        }
      }
    ]
  }
}