Efficiently query on first two digits of indexed int column in MySQL - mysql

Question

I have a table (MySQL 8.0.26, InnoDB) containing an indexed column of MEDIUMINTs that denote the date a record was created:
date_created MEDIUMINT NOT NULL
INDEX idx_created (date_created)
E.g., the entry "210516" denotes 2021-05-16.
Are the following queries roughly equally efficient in utilizing the index?
WHERE 210000<=date_created AND date_created<220000,
WHERE date_created DIV 10000 = 21,
WHERE date_created LIKE '21%', and
WHERE LEFT(date_created, 2) = '21'
I am currently using WHERE date_created DIV 10000 = 21 in my code but wonder if I should alter all queries to make them more efficient.
Thanks a lot in advance.

Answer 1

Look at the type column in EXPLAIN. If it says "ALL" it means it must do a table-scan of all the rows, evaluating the condition expression for each row. This is not using the index.
mysql> explain select * from mytable where 21000<=date_created and date_created < 22000;
+----+-------------+---------+------------+-------+---------------+--------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+--------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | mytable | NULL | range | date_created | date_created | 4 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+---------+------------+-------+---------------+--------------+---------+------+------+----------+-----------------------+
mysql> explain select * from mytable where date_created like '21%';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | mytable | NULL | ALL | date_created | NULL | NULL | NULL | 8192 | 11.11 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
mysql> explain select * from mytable where date_created div 10000 = 21;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 8192 | 100.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
mysql> explain select * from mytable where left(date_created, 2) = '21';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | mytable | NULL | ALL | NULL | NULL | NULL | NULL | 8192 | 100.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
MySQL 8.0 supports expression indexes, which helps a couple of the cases:
mysql> alter table mytable add index expr1 ((left(date_created, 2)));
mysql> explain select * from mytable where left(date_created, 2) = '21';
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| 1 | SIMPLE | mytable | NULL | ref | expr1 | expr1 | 11 | const | 1402 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
mysql> alter table mytable add index expr2 ((date_created DIV 10000));
mysql> explain select * from mytable where date_created div 10000 = 21;
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| 1 | SIMPLE | mytable | NULL | ref | expr2 | expr2 | 5 | const | 1402 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
But expression indexes won't help the LIKE '21%' search, because you'd have to hard-code the value '21%' in the expression for the index definition. You could use that index to search for that value only, not for the value of a different year.