Very simple MySQL index query running very slowly - mysql

Question

I have a very simple query that is running extremely slowly despite being indexed.
My table is as follows:
mysql> show create table mytable
CREATE TABLE `mytable` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`start_time` datetime DEFAULT NULL,
`status` varchar(64) DEFAULT NULL,
`user_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `ix_status_user_id_start_time` (`status`,`user_id`,`start_time`),
### other columns and indices, not relevant
) ENGINE=InnoDB AUTO_INCREMENT=115884841 DEFAULT CHARSET=utf8
Then the following query takes more than 10 seconds to run:
select id from mytable USE INDEX (ix_status_user_id_start_time) where status = 'running';
There are about 7 million rows in the table, and approximately 200 of rows have status running.
I would expect this query to take less than a tenth of a second. It should find the first row in the index with status running. And then scan the next 200 rows until it finds the first non-running row. It should not need to look outside the index.
When I explain the query I get a very strange result:
mysql> explain select id from mytable USE INDEX (ix_status_user_id_start_time) where status =
'running';
+----+-------------+---------+------------+------+------------------------------+------------------------------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+------------------------------+------------------------------+---------+-------+---------+----------+-------------+
| 1 | SIMPLE | mytable | NULL | ref | ix_status_user_id_start_time | ix_status_user_id_start_time | 195 | const | 2118793 | 100.00 | Using index |
+----+-------------+---------+------------+------+------------------------------+------------------------------+---------+-------+---------+----------+-------------+
It is estimating a scan of more than 2 million rows! Also, the cardinality of the status index does not seem correct. There are only about 5 or 6 different statuses, not 344.
Other info
There are somewhat frequent insertions and updates to this table. About 2 rows inserted per second, and 10 statuses updated per second. I don't know how much impact this has, but I would not expect it to be 30 seconds worth.
If I query by both status and user_id, sometimes it is fast (sub 0.1s) and sometimes it is slow (> 1s), depending on the user_id. This does not seem to depend on the size of the result set (some users with 20 rows are quick, others with 4 are slow)
Can anybody explain what is going on here and how it can be fixed?
I am using mysql version 5.7.33

Answer 1

As already mentioned in the comment, you are using many indexes on a big table. So the required memory for this indexes is very high.
You can increase the index buffer size in the my.cnf by changing the innodb_buffer_pool_size to a higher value.
But probably it is more efficient to use less indexes and do not use combined indexes if not absolutely needed.
My guess is, that if you remove all indexes and create only one on status this query will run in under 1s.