Re: Query with 2 x subquery

[Rick James]

I'm not clear on what you are doing, but maybe this:

    SELECT  c1.varid, c1.val, c1.sessionid
        FROM  data_i16 AS c1
        JOIN  
          ( SELECT  varid, MAX(sessionid) AS sessionid
                FROM  data_i16
                WHERE partid = '5'
                  AND sessionid < 1611659
                GROUP BY  varid
          ) AS c2 USING (varid, sessionid)
        WHERE partid = '5'
          AND sessionid < 1611659
        ORDER BY  c1.sessionid DESC;

data_i16 will need INDEX(varid, sessionid) and INDEX(partid, sessionid).

That's "groupwise max" with an added restraing of WHERE partid = '5' AND sessionid < 1611659

http://mysql.rjweb.org/doc.php/groupwise_max