在MySQL中使用max和where子句与子查询进行内部联接

Question

SELECT a.ts, b.barcodenumber, a.remarks, c.department 
FROM documentlog a 
INNER JOIN (select docid, max(logid) as logid from documentlog GROUP BY docid) d ON d.docid=a.docid AND d.logid=a.logid 
INNER JOIN user c ON c.uid=a.user 
INNER JOIN document b ON b.id=a.docid
WHERE c.department = 'PTO' AND b.end = 0

My problem is When I execute this query it's slow like 2sec+ execution but the data is only 9 , How can I speed up the execution of my query?

Old SS for EXPLAIN RESULT

UPDATED SS for EXPLAIN RESULT (Add INDEX logid,docid)

Answer 1

Check out your EXPLAIN result. Notice that MySQL does not use any kind of key when querying the documentlog table ie, the documentlog table does not have a key defined on it. More than 2 million records are processed at this point in your query. This could be the most likely source of the slowness of your query.

Add an index on the docid , and logid fields in your documentlog table and check if it improves the queries' execution time.

Update!!

The output of the updated EXPLAIN query is saying that it is using a full table scan!! (ie, type=ALL ) to produce the output of the main outer query. Why? This is caused by the fact that there are no indices defined on the attributes used in the Where clause ie, ( department and end ).

In general, if you want to speed up queries, then one has to make sure that appropriate indices are defined for the attributes used in the queries' WHERE condition.

By the way, you can learn more about the meaning of MySQL's EXPLAIN result by reading its documentation .