Why does SQL query visit all rows and is very slow - mysql

Question

The table 'reading' contains readings taken every 40s, for today. The query returns averages for 180s periods. 'time_stamp' is indexed. The query below returns a reasonable number of rows (a few hundred) but visits ALL rows and get slower the bigger the table gets. WHERE clause does not seem to be restricting it to today's rows only.
EXPLAIN SELECT
DATE_FORMAT(time_stamp, '%Y-%m-%dT%T+00:00') ,
AVG(temp_c)
FROM reading
WHERE DATE(time_stamp) = CURDATE()
GROUP BY round(UNIX_TIMESTAMP(time_stamp) / 180)
Table schema:
CREATE TABLE reading (
id bigint(20) NOT NULL AUTO_INCREMENT,
time_stamp timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
temp_c float NOT NULL,
pressure_hpa float NOT NULL,
wind_speed_kt int(11) NOT NULL,
wind_dir_degree int(11) NOT NULL,
rain_mm float NOT NULL,
rain_day_mm float NOT NULL,
wind_gust_kt int(11) NOT NULL,
humidity float DEFAULT NULL,
PRIMARY KEY (id),
KEY time_stamp (time_stamp),
KEY time_stamp_idx (time_stamp)
) ENGINE=InnoDB AUTO_INCREMENT=1747097 DEFAULT CHARSET=latin1;

Answer 1

EXPLAIN SELECT
DATE_FORMAT(time_stamp, '%Y-%m-%dT%T+00:00') ,
AVG(temp_c)
FROM reading
WHERE DATE(time_stamp) = CURDATE()
GROUP BY round(UNIX_TIMESTAMP(time_stamp) / 180)
When the above query is executed, MySQL optimizer isn't interested in index scan (could be because of cost factor) rather full table scan is initiated and the issue appears to be because of WHERE DATE(time_stamp) = CURDATE().
Having changed your where clause to time_stamp >= CURDATE(), I've seen index being used and less number of rows were fetched shunning full scan.
Hence, your final query will be:
EXPLAIN SELECT
DATE_FORMAT(time_stamp, '%Y-%m-%dT%T+00:00') ,
AVG(temp_c)
FROM reading
WHERE time_stamp >= CURDATE()
GROUP BY round(UNIX_TIMESTAMP(time_stamp) / 180);
I suspect date(time_stamp) isn't that efficient with index. Similar topic was discussed here (see ypercube's answer).
The above query can be further improved by choosing an alternate of round(UNIX_TIMESTAMP(time_stamp) / 180) as UNIX_TIMESTAMP(timestamp) doesn't use index. But, I'm not trying furthermore.
Hope this helps!