Mysql rand random query record efficiency

I always thought that mysql would randomly query several pieces of data.
SELECT * FROM 'table' order by rand () LIMIT 5

You can.
However, the test results show that the efficiency is very low. It takes more than 8 seconds to query 5 data records in a database with more than 0.15 million entries

According to the official manual, rand () is executed multiple times in the order by clause, which is naturally inefficient and inefficient.

You cannot use a column with RAND () values in an order by clause, because order by wowould evaluate the column multiple times.

Search for Google. Basically, data is randomly obtained by querying max (id) * rand () on the Internet.
SELECT *
FROM 'table' AS t1 JOIN (select round (RAND () * (select max (id) FROM 'table') AS id) AS t2
WHERE t1.id> = t2.id
Order by t1.id asc limit 5;



The preceding statement uses JOIN, Which is used on the mysql forum.
SELECT *
FROM 'table'
WHERE id> = (select floor (MAX (id) * RAND () FROM 'table ')
Order by id LIMIT 1;

I tested it. It took 0.5 seconds and the speed was good, but there was still a big gap with the above statements. I always feel that something is abnormal.

So I changed the statement.
SELECT * FROM 'table'
WHERE id> = (SELECT floor (RAND () * (select max (id) FROM 'table ')))
Order by id LIMIT 1;
The query efficiency is improved, and the query time is only 0.01 seconds.

Finally, complete the statement and add the MIN (id) judgment. At the beginning of the test, because I did not add the MIN (id) Judgment, half of the time is always the first few rows in the table.
Complete query statement:
SELECT * FROM 'table'
WHERE id> = (SELECT floor (RAND () * (select max (id) FROM 'table')-(select min (id) FROM 'table ')) + (select min (id) FROM 'table') order by id LIMIT 1;

SELECT *
FROM 'table' AS t1 JOIN (select round (RAND () * (select max (id) FROM 'table')-(select min (id) FROM 'table ')) + (select min (id) FROM 'table') AS id) AS t2
WHERE t1.id> = t2.id
Order by t1.id LIMIT 1; -- by www.jbxue.com

Finally, these two statements are queried 10 times in php respectively,
The former takes 0.147433 seconds.
The latter takes 0.015130 seconds.
It seems that using the JOIN syntax is much more efficient than using functions directly in the WHERE clause.