How to speed up my mysql query with group by? - mysql

Question

Hi all I have this table schema
create table user_activities
(
id int unsigned auto_increment
primary key,
user_id int unsigned not null,
other_user_id int unsigned not null,
activity_type_id tinyint unsigned not null,
reason varchar(255) null,
created_at timestamp default CURRENT_TIMESTAMP not null,
updated_at timestamp default CURRENT_TIMESTAMP not null on update CURRENT_TIMESTAMP,
constraint user_activities_user_id_other_user_id_unique
unique (user_id, other_user_id),
constraint user_activities_other_user_id_foreign
foreign key (other_user_id) references users (id),
constraint user_activities_activity_type_id_foreign
foreign key (activity_type_id) references user_activity_types (id),
constraint user_activities_user_id_foreign
foreign key (user_id) references users (id)
)
engine = InnoDB
collate = utf8_unicode_ci;
create index user_activities_other_user_id_index
on user_activities (other_user_id);
create index user_activities_activity_type_id_index
on user_activities (activity_type_id);
create index user_activities_user_id_index
on user_activities (user_id);
The table has now 6515846 rows
Goal
I want to write a query to get the users that had the most recent activity in the last 7 days.
I need rows of user_id, mostrecentuseractivitydate
Then in the code I will do some action on them.
My query at the moment is
select updated_at, user_id from user_activities
where created_at > '2022-08-08 15:16:55'
group by user_id
order by max(updated_at) desc
limit 10;
The explain statement result is
1,SIMPLE,user_activities,,index,"user_activities_user_id_other_user_id_unique,user_activities_user_id_index",user_activities_user_id_index,4,,6416255,33.33,Using where; Using temporary; Using filesort
Problem
The query above with the given schema and number of rows takes forever like 5 minutes... and sometimes I receive no response and query hangs forever
That is not acceptable for my requirement.
Any ideas how to speed that up ?
I have already foreign_key as you can see from table schema on the user_id field and the innodb I think also generates index automatically on the foreign key.
I am also adding the where created_at > clause to reduce only the the items in the 7 days.
I even tried without adding the where created_at and did not change much to be honest.
Anyway I am interested only in the data from last 7 days so that where clause can stay

Answer 1

You need an index on created_at. Others might be useful as well, but start there.

Answer 2

You want to have a composite key with user_id and created_at in it.
This key should make it possible to do the group by as well as the where clause at the same time.
Try this:
create index user_activities_user_id_created_at on user_activities (user_id, created_at);