Mysql random acquisition record _ MySQL

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Mysql random acquisition records

MYSQL Random extraction implementation method. 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.

However, the test results show that the efficiency is very low. It takes more than four seconds for a database with more than 1 million entries to query one data entry. 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 instanceprice_attach where id <= (select floor (RAND () * (select max (ID) FROM INSTANCEPRICE_ATTACH) order by id limit 1

This is about 95 ms.

The original write is> =, which is always the record with the largest id.

Someone handles this to retrieve duplicate records:

SELECT t1 .*

FROM tablename AS t1 JOIN

(Select round (RAND () * (select max (id) FROM tablename)-(select min (id) FROM tablename) +

(Select min (id) FROM tablename) AS id) AS t2

WHERE t1.id> = t2.id

Order by t1.id LIMIT 1;

SELECT * FROM tablename

WHERE id> = (SELECT floor (RAND () * (select max (id) FROM tablename )-

(Select min (id) FROM tablename) + (select min (id) FROM tablename )))

Order by id LIMIT 1;

It takes more than 90 milliseconds. the first write method is faster on the Internet, but it seems no difference in my test.

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