Re: Alternative for IN+Subselect in this case

[Rick James]

# what you have:
SELECT  col1, col2
    FROM  table b
    WHERE  b.id IN (
        SELECT  id
            FROM  idxmail i
            JOIN  matching m ON i.proxyaddress = m.proxyaddress
            WHERE  m.mailnickname = '@var1'); 
# Plan A:
SELECT  col1, col2
    FROM  table b
    JOIN  idxmail i ON i.id = b.id
    JOIN  matching m ON i.proxyaddress = m.proxyaddress
    WHERE  m.mailnickname = '@var1'; 
# Plan B:
SELECT  col1, col2
    FROM  table b
    JOIN  
      ( SELECT  id
            FROM  idxmail i
            JOIN  matching m ON i.proxyaddress = m.proxyaddress
            WHERE  m.mailnickname = '@var1'
      ) x ON x.id = b.id;

You need
b: INDEX(id) (or PRIMARY KEY(id))
i: INDEX(proxyaddress)
m: INDEX(mailnickname, proxyaddress)

If you don't understand the "compound" index I am suggesting for m, please study
http://mysql.rjweb.org/doc.php/index1