Mysql randomly queries several data entries

To query five non-repeated data records in mysql, use the following: SELECT * FROM 'table' ORDERBYRAND () LIMIT5. However, the test results show that the efficiency is very low. It takes more than 8 seconds to search Google for more than 5 data records in a database with more than 0.15 million records. on the Internet, it is basically a query of max (id) * ran

To query five non-duplicate data records in mysql, use the following: SELECT * FROM 'table' order by rand () LIMIT 5. However, the test results show that the efficiency is very low. It takes more than 8 seconds to search Google for more than 5 data records in a database with more than 0.15 million records. on the Internet, it is basically a query of max (id) * ran

To query five non-duplicate data records in mysql, use the following:

SELECT * FROM 'table' ORDERBYRAND () LIMIT5

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

Search for Google. Basically, data is randomly obtained by querying max (id) * rand () on the Internet.

SELECT *
FROM 'table' ASt1JOIN (SELECTROUND ( RAND () * (SELECTMAX (id) FROM 'table') ASid) ASt2
WHEREt1.id> = t2.id
ORDERBYt1.idASCLIMIT5;

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 preceding statement uses JOIN, Which is used on the mysql forum.

SELECT *
FROM 'table'
WHEREid> = (SELECTFLOOR (MAX (id )* RAND () FROM 'table ')
ORDERBYidLIMIT1;

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'
WHEREid> = (SELECTfloor ( RAND () * (SELECTMAX (id) FROM 'table ')))
ORDERBYidLIMIT1;

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'
WHEREid> = (SELECTfloor ( RAND () * (SELECTMAX (id) FROM 'table')-(SELECTMIN (id) FROM 'table') + (SELECTMIN (id) FROM 'table ')))
ORDERBYidLIMIT1;

SELECT *
FROM 'table' ASt1JOIN (SELECTROUND ( RAND () * (SELECTMAX (id) FROM 'table')-(SELECTMIN (id) FROM 'table') + (SELECTMIN (id) FROM 'table') ASid) ASt2
WHEREt1.id> = t2.id
ORDERBYt1.idLIMIT1;

Finally, the two statements are queried 10 times 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.