Mysql strange behaviour on querying tinyint boolean field - mysql

Question

I have a mysql table voucher_codes with the following schema:
mysql> describe voucher_codes;
+--------------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------------+------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| user_id | int(10) unsigned | YES | MUL | NULL | |
| promo_code | varchar(30) | YES | MUL | NULL | |
| voucher_value | double | NO | | NULL | |
| date_created | datetime(6) | NO | | NULL | |
| date_modified | datetime(6) | NO | | NULL | |
| redeem_flag | tinyint(1) | NO | | NULL | |
| fk_voucher_rule_id | int(11) | NO | MUL | NULL | |
+--------------------+------------------+------+-----+---------+----------------+
The indexes defined on the table are as follows:
mysql> show index from voucher_codes;
+---------------+------------+--------------------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------------+------------+--------------------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| voucher_codes | 0 | PRIMARY | 1 | id | A | 15280692 | NULL | NULL | | BTREE | | |
| voucher_codes | 1 | voucher_codes_user_id | 1 | user_id | A | 2805369 | NULL | NULL | YES | BTREE | | |
| voucher_codes | 1 | voucher_codes_promo_code | 1 | promo_code | A | 7389780 | NULL | NULL | YES | BTREE | | |
| voucher_codes | 1 | fk_voucher_rule_id | 1 | fk_voucher_rule_id | A | 60 | NULL | NULL | | BTREE | | |
+---------------+------------+--------------------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
The numbers of records with voucher_code 1 and redeem_flag as 0 and 1 are as follows:
mysql> select count(*) from voucher_codes where redeem_flag =0 and fk_voucher_rule_id=1;
+----------+
| count(*) |
+----------+
| 135114 |
+----------+
1 row in set (3.17 sec)
mysql> select count(*) from voucher_codes where redeem_flag =1 and fk_voucher_rule_id=1;
+----------+
| count(*) |
+----------+
| 1575024 |
+----------+
1 row in set (2.56 sec)
There are 1.5+ million records with redeem_flag as 1 and 100k+ are 0.
But the query with redeem_flag as 1 takes less time as compared with query with redeem_flag 0.
Below are the results:
mysql> select promo_code from voucher_codes where redeem_flag =0 and fk_voucher_rule_id=1 limit 1 ;
+--------------+
| promo_code |
+--------------+
| XXXXXXXXXX |
+--------------+
1 row in set (3.67 sec)
mysql> explain select promo_code from voucher_codes where redeem_flag =0 and fk_voucher_rule_id=1 limit 1 ;
+----+-------------+---------------+------------+------+--------------------+--------------------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------+------------+------+--------------------+--------------------+---------+-------+---------+----------+-------------+
| 1 | SIMPLE | voucher_codes | NULL | ref | fk_voucher_rule_id | fk_voucher_rule_id | 4 | const | 3258352 | 10.00 | Using where |
+----+-------------+---------------+------------+------+--------------------+--------------------+---------+-------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
Query with redeem_flag as 1:
mysql> select promo_code from voucher_codes where redeem_flag =1 and fk_voucher_rule_id=1 limit 1 ;
+------------+
| promo_code |
+------------+
| XXXXXXXXX |
+------------+
1 row in set (0.00 sec)
mysql> explain select promo_code from voucher_codes where redeem_flag =1 and fk_voucher_rule_id=1 limit 1 ;
+----+-------------+---------------+------------+------+--------------------+--------------------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------------+------------+------+--------------------+--------------------+---------+-------+---------+----------+-------------+
| 1 | SIMPLE | voucher_codes | NULL | ref | fk_voucher_rule_id | fk_voucher_rule_id | 4 | const | 3258352 | 10.00 | Using where |
+----+-------------+---------------+------------+------+--------------------+--------------------+---------+-------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
Unable to understand this behaviour. Even though the index fk_voucher_rule_id is getting used, but it is taking more time with records which are more as compared to less number of records.
Update: Adding show create table result:
mysql> show create table voucher_codes;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| voucher_codes | CREATE TABLE `voucher_codes` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(10) unsigned DEFAULT NULL,
`promo_code` varchar(30) DEFAULT NULL,
`voucher_value` double NOT NULL,
`date_created` datetime(6) NOT NULL,
`date_modified` datetime(6) NOT NULL,
`redeem_flag` tinyint(1) NOT NULL,
`fk_voucher_rule_id` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `voucher_codes_user_id` (`user_id`),
KEY `voucher_codes_promo_code` (`promo_code`),
KEY `fk_voucher_rule_id` (`fk_voucher_rule_id`)
) ENGINE=InnoDB AUTO_INCREMENT=16305657 DEFAULT CHARSET=latin1 |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Answer 1

First of all, test speed of your queries using SELECT SQL_NO_CACHE ... instead of just SELECT .... It is quite odd that query runs in 0.00 time.
Furthermore, create a composite index on redeem_flag, fk_voucher_rule and then retest your queries for comparison.
NOTE: You should use same order of fields in a query as it is in your index. Also, as #Raymod Nijland said, remove redundant indices. For example, if you have index fk_voucher_rule, redeem_flag it will be used both for queries that use only fk_voucher_rule and fk_voucher_rule, redeem_flag queries (other way around does not apply, you can't use it only for querying redeem_flag).