Mysql is looking in much more, estimate rows, then expected - mysql

Question

I have user_rates table where i have two user foreign references user_id_owner and user_id_rated.
This is my create table query:
CREATE TABLE `user_rates` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`user_id_owner` int(10) unsigned NOT NULL,
`user_id_rated` int(10) unsigned NOT NULL,
`value` int(11) NOT NULL COMMENT '0 - dislike, 1 - like',
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `user_rates_user_id_rated_foreign` (`user_id_rated`),
KEY `user_rates_user_id_owner_foreign` (`user_id_owner`),
CONSTRAINT `user_rates_user_id_owner_foreign` FOREIGN KEY (`user_id_owner`) REFERENCES `users` (`id`),
CONSTRAINT `user_rates_user_id_rated_foreign` FOREIGN KEY (`user_id_rated`) REFERENCES `users` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1825767 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
When i execute this query:
EXPLAIN SELECT
user_id_rated
FROM
`user_rates` AS ur
WHERE
ur.user_id_owner = 10101;
It shows estimate rows to examine 107000, but returning only 60000.
Can you explain me why it's examining so many rows, when it is comparing with equality operator and also comparing field is foreign key?
EDIT
I am getting this on EXPLAIN
I want to add several where clauses also. At last my query looks like this:
Explain SELECT
user_id_rated
FROM
`user_rates` AS ur
WHERE
ur.user_id_owner = 10101
AND (ur.value IN (1, 2, 3)
OR (ur.value = 0
AND ur.created_at > '2020-02-04 00:00:00'));
Output:
It will be nice if query can be more optimized. I don't understand why isn't it reducing estimate rows.
Steps i tried when optimizing
Added compose index on (user_id_owner, value, created_at)
But estimate row is not reducing, It is filtering even more rows
Maybe i am doing indexing wrong? I really don't know how to make proper indexes. Sorry for bad question, I am new here. Thanks in advance.

Answer 1

The "rows" is an estimate, often quite far off -- sometimes even worse than your example. The incorrectness of the estimate rarely impacts performance.
You can run ANALYZE TABLE tablename to improve the estimate. But it may still not be better.
For the current query, use:
( SELECT user_id_rated
FROM `user_rates` AS ur
WHERE ur.user_id_owner = 10101
AND ur.value IN (1, 2, 3)
)
UNION ALL
( SELECT user_id_rated
FROM `user_rates` AS ur
WHERE ur.user_id_owner = 10101
AND ur.value = 0
AND ur.created_at > '2020-02-04 00:00:00'
);
And have the composite (and "covering") indexes:
INDEX(user_id_owner, value, user_id_rated)
INDEX(user_id_owner, value, created_at, user_id_rated)
If there are other variations of the query, show us. As you may guess; the details are important.
(The simplified version of the query does not provide any useful information when discussing the real query.)