SELECT 
  cscart_product_questions.*, 
  cscart_profile_fields_data.value as username 
FROM 
  cscart_product_questions 
  LEFT JOIN cscart_profile_fields_data ON cscart_product_questions.user_id = cscart_profile_fields_data.object_id 
WHERE 
  product_id = 62270 
  AND cscart_profile_fields_data.object_type = 'U' 
  AND cscart_profile_fields_data.field_id = 60 
ORDER BY 
  cscart_product_questions.timestamp ASC

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.90"
    },
    "ordering_operation": {
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "1.00"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_product_questions",
            "access_type": "ALL",
            "rows_examined_per_scan": 10,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.45",
              "eval_cost": "0.10",
              "prefix_cost": "1.55",
              "data_read_per_join": "32"
            },
            "used_columns": [
              "question_id",
              "timestamp",
              "user_id",
              "product_id",
              "question_text"
            ],
            "attached_condition": "(`cscartdb`.`cscart_product_questions`.`product_id` = 62270)"
          }
        },
        {
          "table": {
            "table_name": "cscart_profile_fields_data",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "object_id",
              "object_type",
              "field_id"
            ],
            "key_length": "9",
            "ref": [
              "cscartdb.cscart_product_questions.user_id",
              "const",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "index_condition": "(`cscartdb`.`cscart_product_questions`.`user_id` = `cscartdb`.`cscart_profile_fields_data`.`object_id`)",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.10",
              "prefix_cost": "1.90",
              "data_read_per_join": "784"
            },
            "used_columns": [
              "object_id",
              "object_type",
              "field_id",
              "value"
            ]
          }
        }
      ]
    }
  }
}