MySQL — Query possible without three SELECT? - mysql

Question

Happy New Year's, everyone!
I'll jump right into it. I've inherited a project that includes a very large database. Some tables are upwards of 285.6GiB.
One of the larger tables is user-ratings. The table has the following columns (simplified):
from — VARCHAR(19)
reason — VARCHAR(512)
stars — TINYINT
timestamp — TIMESTAMP
to — VARCHAR(19)
Currently, users can check the ratings of other users. THis shows a summary of their ratings given, received, as well as their entire last 5 ratings received. To do this, we'd currently use the following queries (simplified):
# First query — ratings given from the user
SELECT Avg(`stars`),
Min(`timestamp`),
Count(*),
Count(DISTINCT( `to` ))
INTO avgStarsGiven, firstRatingGivenAt, totalRatingsGiven,
totalUniqueRatingsGiven
FROM `ratings`
WHERE `from` = user;
# Second query — ratings received by the user
SELECT Avg(`stars`),
Min(`timestamp`),
Count(*),
Count(DISTINCT( `from` ))
INTO avgStarsReceived, firstRatingReceivedAt, totalRatingsReceived,
totalUniqueRatingsReceived
FROM `ratings`
WHERE `to` = user;
# Third query — get the last 5 ratings to the user
SELECT * FROM `ratings` WHERE `to` = user ORDER BY `timestamp` DESC LIMIT 5;
Is it possible to retrieve all of this information without having to go over the entire table 3 times?
Thanks in advance!
Edit: The table and version are below:
# 8.0.27-0ubuntu0.20.04.1
CREATE TABLE `ratings` (
`no` int NOT NULL AUTO_INCREMENT,
`from` varchar(19) NOT NULL,
`to` varchar(19) NOT NULL,
`reason` varchar(512) NOT NULL,
`stars` tinyint NOT NULL,
`timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`no`),
KEY `i-ratings-from` (`from`) /*!80000 INVISIBLE */,
KEY `i-ratings-to` (`to`) /*!80000 INVISIBLE */,
KEY `i-ratings-from-to-timestamp` (`from`,`to`,`timestamp`),
CONSTRAINT `fk-ratings-from` FOREIGN KEY (`from`) REFERENCES `users` (`user`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `fk-ratings-to` FOREIGN KEY (`to`) REFERENCES `users` (`user`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=59 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

Answer 1

You probably don't need to go over the entire table in any of these queries, if you define indexes on the columns from and to. When you look for a person by name in a telephone book, do you read the entire book every time?
ALTER TABLE ratings
ADD INDEX (`from`),
ADD INDEX (`to`, `timestamp`);
You can use EXPLAIN to confirm that it's using the index:
EXPLAIN SELECT * FROM `ratings` WHERE `to` = <example-value>
ORDER BY `timestamp` DESC LIMIT 5;
The EXPLAIN report should show you in its rows field that it will examine a small subset of the rows of the table. This is one of the benefits of an index, to narrow down the search efficiently, so a query doesn't need to scan the entire table.
You edited your question above to add the CREATE TABLE definition.
I see that your table already has some indexes, but these indexes aren't tailored very well to the queries you show. You might like to review my presentation How to Design Indexes, Really, or the video.
Also I see that some of your indexes are defined with the INVISIBLE option, which means the optimizer won't use these indexes. Read https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html for details.