MySQL 使用 JOIN 和 COUNT 进行查询优化

Question

I have the following MySQL Query:

SELECT t1.id, t1.releaseid, t1.site, t1.date, t2.pos FROM `tracers` as t1
LEFT JOIN (
    SELECT `releaseid`, `date`, COUNT(*) AS `pos` 
    FROM `tracers` GROUP BY `releaseid`
) AS t2 ON t1.releaseid = t2.releaseid AND t2.date <= t1.date 
ORDER BY `date` DESC , `pos` DESC LIMIT 0 , 100

The idea being to select a release and count how many other sites had also released it prior to the recorded date, to get the position.

Explain says:

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   PRIMARY t1  ALL NULL    NULL    NULL    NULL    498422  Using temporary; Using filesort
1   PRIMARY <derived2>  ALL NULL    NULL    NULL    NULL    91661    
2   DERIVED tracers index   NULL    releaseid   4   NULL    498422   

Any suggestions on how to eliminate the Using temporary; Using filesort? It takes a loooong time. The indexes I have thought of and tried haven't helped anything.

Answer 1

This answer below maybe not change explain output, however if your major problem is sorting data, which it identified by removing order clause will makes your query run faster , try to sort your subquery join table first and your query will be:

SELECT t1.id, t1.releaseid, t1.site, t1.date, t2.pos FROM `tracers` as t1
LEFT JOIN (
    SELECT `releaseid`, `date`, COUNT(*) AS `pos` 
    FROM `tracers` GROUP BY `releaseid`
    ORDER BY `pos` DESC -- additional order
) AS t2 ON t1.releaseid = t2.releaseid AND t2.date <= t1.date 
ORDER BY `date` DESC , `pos` DESC LIMIT 0 , 100

Note: My db version is mysql-5.0.96-x64, maybe in another version you get different result.

Answer 2

Try adding an index on tracers.releaseid and one on tracers.date

Answer 3

1. make sure you have an index on releaseid.
2. flip your JOIN, the sub-query must be on the left side in the LEFT JOIN.
3. put the ORDER BY and LIMIT clauses inside the sub-query.

Answer 4

Try having two indices, one on (date) and one on (releaseid, date) .

Another thing is that your query does not seem to be doing what you describe it does. Does it actually count correctly?

Try rewriting it as:

SELECT t1.id, t1.releaseid, t1.site, t1.`date`
     , COUNT(*) AS pos
FROM tracers AS t1
  JOIN tracers AS t2
    ON  t2.releaseid = t1.releaseid
    AND t2.`date` <= t1.`date` 
GROUP BY t1.releaseid
ORDER BY t1.`date` DESC
       , pos DESC
LIMIT 0 , 100

or as:

SELECT t1.id, t1.releaseid, t1.site, t1.`date`
     , ( SELECT COUNT(*)
         FROM tracers AS t2
         WHERE t2.releaseid = t1.releaseid
           AND t2.`date` <= t1.`date`
       ) AS pos
FROM tracers AS t1
ORDER BY t1.`date` DESC
       , pos DESC
LIMIT 0 , 100