How can I make this mysql query faster? Is the order a killer? The limit?

Question

SELECT *  FROM table WHERE city LIKE example ORDER by RAND() Limit 10

I'm trying to return, obv, some random entries from a table, but it's on quite a few pages that get hit so it can cause the site to slow down. I comment out the query and all is well again so I know it's this, but I'd like to take this opportunity to learn a bit about how to do queries better.

I thought an index might help, but where there's no joins that seems unhelpful in this case. Thanks.

Answer 1

You can make this faster by adding an index on table(city) .

I think the query would be clearer using = rather than like :

SELECT t.* 
FROM table t
WHERE t.city = example
ORDER by RAND()
Limit 10;

Depending on how many rows are returned, there might be further optimizations.

For instance, one method is:

SELECT t.* 
FROM (SELECT t.*, RAND() as rnd, (@rn := @rn + 1) as rn
      FROM table t CROSS JOIN
           (SELECT @rn := 0) params
      WHERE t.city = example
     ) t 
WHERE rnd < (100 / @rn)  -- get a sample of about 100 records
ORDER BY rnd
LIMIT 10;

This uses variables and the WHERE to limit the rows to about 100 before sorting. That sort should be pretty fast.