MySql calendar table and performances - mysql

Question

for a project i'm working on, i have a single table with two dates meaning a range of dates and i needed a way to "multiply" my rows for every day in between the two dates.
So for instance i have start 2017-07-10, end 2017-07-14
I needed to have 4 lines with 2017-07-10, 2017-07-11, 2017-07-12, 2017-07-13
In order to do this i found here someone mentioning using a "calendar table" with all the dates for years.
So i built it, now i have these two simple tables:
CREATE TABLE `time_sample` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`start` varchar(16) DEFAULT NULL,
`end` varchar(16) DEFAULT NULL,
PRIMARY KEY (`societa_id`),
KEY `start_idx` (`start`),
KEY `end_idx` (`end`)
) ENGINE=MyISAM AUTO_INCREMENT=222 DEFAULT CHARSET=latin1;
This table contains my date ranges, start and end are indexed, the primary key is an incremental int.
Sample Row:
id start end
1 2015-05-13 2015-05-18
Second table:
CREATE TABLE `time_dimension` (
`id` int(11) NOT NULL,
`db_date` date NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `td_dbdate_idx` (`db_date`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
This has a date indexed for every day for many years to come.
Sample row:
id db_date
20120101 2012-01-01
Now, i made the join:
select * from time_sample s join time_dimension t on (t.db_date >= start and t.db_date < end);
This takes 3ms. Even if my first table is HUGE, this query will always be very quick (max i've seen was 50ms with a lot of records).
The issue i have is while grouping results (i need them grouped for my application):
select * from time_sample s join time_dimension t on (t.db_date >= start and t.db_date < end) group by db_date;
This takes more than one second with not so many rows in the first table, increasing dramatically. Why is this happening and how can i avoid this?
Changing the data types doesn't help, having the second table with just one column doesn't help.
Can i have suggestions, please :(

Answer 1

I cannot replicate this result...
I have a calendar table with lots of dates: calendar(dt) where dt is a PRIMARY KEY DATE data type.
DROP TABLE IF EXISTS time_sample;
CREATE TABLE time_sample (
id int(11) NOT NULL AUTO_INCREMENT,
start date not NULL,
end date null,
PRIMARY KEY (id),
KEY (start,end)
);
INSERT INTO time_sample (start,end) VALUES ('2010-03-13','2010-05-09);
SELECT *
FROM calendar x
JOIN time_sample y
ON x.dt BETWEEN y.start AND y.end;
+------------+----+------------+------------+
| dt | id | start | end |
+------------+----+------------+------------+
| 2010-03-13 | 1 | 2010-03-13 | 2010-05-09 |
| 2010-03-14 | 1 | 2010-03-13 | 2010-05-09 |
| 2010-03-15 | 1 | 2010-03-13 | 2010-05-09 |
| 2010-03-16 | 1 | 2010-03-13 | 2010-05-09 |
...
| 2010-05-09 | 1 | 2010-03-13 | 2010-05-09 |
+------------+----+------------+------------+
58 rows in set (0.10 sec)
EXPLAIN
SELECT * FROM calendar x JOIN time_sample y ON x.dt BETWEEN y.start AND y.end;
+----+-------------+-------+--------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------+------+--------------------------+
| 1 | SIMPLE | y | system | start | NULL | NULL | NULL | 1 | |
| 1 | SIMPLE | x | range | PRIMARY | PRIMARY | 3 | NULL | 57 | Using where; Using index |
+----+-------------+-------+--------+---------------+---------+---------+------+------+--------------------------+
2 rows in set (0.00 sec)
Even with a GROUP BY, I'm struggling to reproduce the problem. Here's a simple COUNT...
SELECT SQL_NO_CACHE dt, COUNT(1) FROM calendar x JOIN time_sample y WHERE x.dt BETWEEN y.start AND y.end GROUP BY dt ORDER BY COUNT(1) DESC LIMIT 3;
+------------+----------+
| dt | COUNT(1) |
+------------+----------+
| 2010-04-03 | 2 |
| 2010-05-05 | 2 |
| 2010-03-13 | 2 |
+------------+----------+
3 rows in set (0.36 sec)
EXPLAIN
SELECT SQL_NO_CACHE dt, COUNT(1) FROM calendar x JOIN time_sample y WHERE x.dt BETWEEN y.start AND y.end GROUP BY dt ORDER BY COUNT(1) DESC LIMIT 3;
+----+-------------+-------+-------+---------------+---------+---------+------+---------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+---------+----------------------------------------------+
| 1 | SIMPLE | y | index | start | start | 7 | NULL | 2 | Using index; Using temporary; Using filesort |
| 1 | SIMPLE | x | index | PRIMARY | PRIMARY | 3 | NULL | 1000001 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+---------+----------------------------------------------+