Mysql optimization method for slow random data retrieval _ MySQL

Mysql optimized the method of slow random data retrieval. bitsCN.com the day before yesterday because of work requirements, I randomly extracted several records from a database with a 5 million records, here, I directly use the mysql rand by function. it doesn't matter if thousands of records exist, but if tens of thousands of records arrive, it will take several seconds, let's take a look at the slow optimization process of getting random data from mysql.
MySQL often needs to obtain random data. for example, to randomly extract a record from the tablename table, the general syntax is:

SELECT * FROM tablename order by rand () LIMIT 1

However, I checked the MYSQL official Manual. the prompt for RAND () indicates that the RAND () function cannot be used in the ORDER BY clause, this will cause the data column to be scanned multiple times. However, in MYSQL 3.23, order by rand () can still be used for random operations.
The test showed that the efficiency was 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;

However, five consecutive records are generated. The solution is to query only one item at a time and query five times. Even so, it is worthwhile because it takes less than 0.15 million seconds to query 0.01 tables.
The following 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.
The complete query statement is:

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 (
# Minimum value + (1 to minimum and maximum value difference)
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;

Finally, the two statements are queried 10 times in php respectively. The former takes 0.147433 seconds, and 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.

SELECT *
FROM 'table' AS t1 JOIN (select round (RAND () * (select max (id) FROM 'table ')