使用IN（）时主索引不起作用吗？

Question

SELECT 
  i.id 
FROM 
  itable i
WHERE 
  i.id IN (
    SELECT 
      itable_id j 
    FROM 
      ijoin j 
    WHERE 
      j.user_id = '1'
  )

This query examines every single row on itable, which is about 3 million. Since i.id is the PRIMARY KEY, shouldn't it just fetch those items?

How can I optimize this query?

Answer 1

MySQL is notoriously poor at optimizing this type of query. Instead of selecting the small number of values in the subquery and using them to index into the main table, it iterates through all the rows in the main query and uses them to index into the table in the subquery.

It does better if you use a JOIN :

SELECT DISTINCT i.id
FROM itable i
JOIN ijoin j ON i.id = j.itable_id
WHERE j.user_id = '1'