Question

So I have this relatively simple query to pull a random row based on a few criteria from two different MyISAM tables:

SELECT 
    table1 . * , table2 . * 
FROM 
    Table1 AS table1
LEFT JOIN 
    Table2 AS table2
USING (
    col1
)
WHERE 
    table1.col1 !=  '5324372'
AND 
    table1.col2 LIKE  'S'
AND (
    table1.col3 LIKE  'I'
    OR table1.col3 LIKE  'V-G'
)
AND (
    table2.col1 =  'A'
    OR table2.col2 =  'B'
)
ORDER BY RAND( ) 
LIMIT 1

…which at the beginning worked fine, but as my Database began to grow, now takes several seconds to execute.

All columns used above are indexed, so it’s not an indexing problem.

From what I’ve researched, it’s due to the ORDER BY RAND() line which apparently runs very slow on large tables.

I’ve seen some potential solutions to this problem, but they are quite cryptic and I could not wrap my head around any of them in relation to my query above.

I’m actually surprised that MySQL does not have a built-in solution to this very slow ORDER BY RAND() problem as it would seem to me like it is a very basic need for many types of applications needing to select a random row.

So how can I optimize my query above so that I get the same/similar effect without the long execution times?

Answer 1

You are choosing only one row , I think you can choose one row randomly by LIMIT randomly. Example:

If you have:

SELECT * table  ORDER BY RAND( )  LIMIT 1;

change it to

SELECT * table    LIMIT $randomvalue  ,$randomvalue + 1;

$randomvalue is a random value chosen from the application level.

update: the following answer is more clear than the above one.

//get the total number of rows
$result= mysql_query(" SELECT  COUNT(*) AS total FROM `table` ");
$row = mysql_fetch_array($result);
$total=$row['total'];


//create random value from 1 to the total of rows 
$randomvalue =rand(1,$total);


//get the random row

$result= mysql_query(" SELECT  * FROM `table` limit $randomvalue,1");