MySQL query performance degrade with filesort - mysql

Question

I have the below table with more than 190M records,
CREATE TABLE notification (
_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
recipient CHAR(11) NOT NULL,
recipient_group CHAR(11),
topic VARCHAR(25) NOT NULL,
identifier VARCHAR(60) NOT NULL,
timestamp TIMESTAMP(3) NOT NULL,
type VARCHAR(255) NOT NULL,
actioned BIT NOT NULL DEFAULT 0,
expiry_timestamp TIMESTAMP DEFAULT NULL,
INDEX recipient_recipient_group_timestamp_id (recipient, recipient_group, timestamp DESC, _id DESC),
INDEX topic_identifier (topic, identifier),
INDEX expiry_timestamp (expiry_timestamp),
UNIQUE recipient_recipient_group_topic_identifier (recipient, recipient_group, topic, identifier)
) CHARACTER SET ascii COLLATE ascii_bin;
Now I want to query all the notifications for a recipient belonging to a group based on the timestamp,
explain
select * from notification
where (recipient = 'recipient' and (recipient_group = 'group' or recipient_group is null)
and (expiry_timestamp > {ts '2018-06-26 08:00:00.0'} or expiry_timestamp is null)
and timestamp > {ts '1970-01-01 00:00:00.0'} and type in ('TYPE'))
order by timestamp desc, _id desc limit 10;
I have noticed that this query perform poorly when there are large number of notifications for a user as MySQL ended up using filesort in order by timestamp and _id.
+----+-------------+--------------+------------+-------------+----------------------------------------------------------------------------------------------------+--------------------------------------------+---------+-------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------------+------------+-------------+----------------------------------------------------------------------------------------------------+--------------------------------------------+---------+-------------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | notification | NULL | ref_or_null | recipient_recipient_group_topic_identifier,recipient_recipient_group_timestamp_id,expiry_timestamp | recipient_recipient_group_topic_identifier | 23 | const,const | 2 | 5.01 | Using index condition; Using where; Using filesort |
+----+-------------+--------------+------------+-------------+----------------------------------------------------------------------------------------------------+--------------------------------------------+---------+-------------+------+----------+----------------------------------------------------+
Is there a way to improve the query performance may be adding/modifying an index?
Edit:
It seems that MySQL uses index recipient_recipient_group_timestamp_id if I remove or recipient_group is null from the where condition.

Answer 1

In general, you can't optimize an inequality condition with an index and also eliminate the filesort in the same query.
Think of a telephone book. It's sorted by last name, first name, then if there are still ties (people with the same name), it's sorted by the phone number. So if you want this query:
SELECT * FROM PhoneBook WHERE last_name=? AND first_name=?
ORDER BY phone_number;
Then the sorting will be a no-op, because if the first two are tied, the matching rows will naturally be stored in the requested order already. The query can skip the filesort if it simply reads the rows in the index order.
But if you query any type of inequality:
SELECT * FROM PhoneBook WHERE last_name=? AND first_name LIKE 'S%'
ORDER BY phone_number;
This matches multiple first names, and reading the matching rows in the index order will not be tied, so they aren't guaranteed to be sorted by phone number. The query has to sort the rows matched.
The same is true of any other type of inequality or range search that can be indexed: !=, IN(), LIKE, >, etc.