如何优化此复杂的MySQL查询

Question

I have this query

SELECT id, number, count, name FROM temp AS t1 WHERE (SELECT COUNT(*) FROM temp AS t2 WHERE t2.number = t1.number AND t2.count > t1.count ) = 0 AND id NOT IN (SELECT id FROM temp WHERE count < 3)

Its currently taking too long to execute, table temp has 150 000 rows and increasing.

I am running this via php (mysqli). How can I improve the performance?

Answer 1

In your query the condition

AND id NOT IN (SELECT id FROM temp WHERE count < 3)

could be used as

and `count` < 3 

You may also convert the subquery with exists strategy and finally adding some indexes. https://dev.mysql.com/doc/refman/5.5/en/subquery-optimization-with-exists.html

select id,number,`count`,name from temp t1
where t1.`count` < 3
and not exists(
 select 1 from temp t2 
 where t2.number = t1.number AND t2.count > t1.count 
)

The indexing may require if not already there as

alter table temp add index cnt_idx(`count`);
alter table temp add index number_idx(`number`);

Make sure to take a backup of the table before applying the index.

You can always use explain select to check the query health.