Mysql – Slow SELECT examining whole table

Can you please help me to understand, why this select examines 828004 rows (whole database) to return 21 rows?

# Time: 161121 14:00:44
# User@Host: root[root] @ localhost [127.0.0.1]
# Query_time: 2.507397  Lock_time: 0.000089 Rows_sent: 21  Rows_examined: 828004
SET timestamp=1479733244;
SELECT DATE(FROM_UNIXTIME(Epoch)) AS NodeDate, DeviceID as NodeID, Description, 
AVG(AD0) AS AverageAD0,AVG(AD1) AS AverageAD1 
FROM Data 
WHERE DATE(FROM_UNIXTIME(Epoch)) >= CURDATE() 
AND HOUR(FROM_UNIXTIME(Epoch)) >= HOUR(current_time()) 
GROUP BY NodeDate, NodeID 
ORDER BY AVG(AD0) DESC, AVG(AD0) DESC;

Data table structure

mysql> DESCRIBE Data;
+-------------+-------------+------+-----+---------+----------------+
| Field       | Type        | Null | Key | Default | Extra          |
+-------------+-------------+------+-----+---------+----------------+
| Id          | int(10)     | NO   | MUL | NULL    | auto_increment |
| Epoch       | int(10)     | NO   |     | NULL    |                |
| Address     | varchar(32) | NO   |     | NULL    |                |
| DeviceID    | varchar(32) | NO   |     | NULL    |                |
| Description | varchar(32) | NO   |     | NULL    |                |
| AD0         | int(10)     | NO   |     | NULL    |                |
| AD1         | int(10)     | NO   |     | NULL    |                |
| AD2         | int(10)     | NO   |     | NULL    |                |
| AnalogZero  | int(8)      | YES  |     | NULL    |                |
| Lat         | varchar(32) | NO   |     | NULL    |                |
| Lon         | varchar(32) | NO   |     | NULL    |                |
| RSSI        | varchar(8)  | NO   |     | NULL    |                |
+-------------+-------------+------+-----+---------+----------------+
12 rows in set (0.00 sec)

Explain

+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra                                        |
+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------+
|  1 | SIMPLE      | Data  | ALL  | NULL          | NULL | NULL    | NULL | 822772 | Using where; Using temporary; Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+--------+----------------------------------------------+

Indexes

mysql> SHOW INDEX FROM Data;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Data  |          1 | Id       |            1 | Id          | A         |      839767 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

The select is used to obtain averages from today AND current hour+.

This query is also examining all rows and looks super-simple. I have a 2-Core VPS and that is not handling such simple queries.

SELECT AVG(AD1) AS AverageAD1,AVG(AD2) AS AverageAD2, AVG(AD0) AS AverageAD0
FROM Data 
WHERE DeviceID="xxxxxxxx" 
AND FROM_UNIXTIME(Epoch) >= NOW() - INTERVAL 60 MINUTE;