使用MAX（id）优化查询

Question

I have this query that shows me the last id by id_units , depending on the date.

My query is:

SELECT max(swg.id) AS id
 FROM swg
 WHERE swg.status != 0
 AND sg.dateswg <= TIMESTAMPADD(DAY,(6-WEEKDAY('2015-12-28')),'2015-12-28')
 GROUP BY swg.id_units
ORDER BY swg.id;

The query returns more than 2000 records, I think that there is a way that I can performance this query.

I am trying to performance my query like this:

SELECT id
FROM (SELECT * FROM swg WHERE status != 0 AND dateswg <= TIMESTAMPADD(DAY,(6-WEEKDAY('2015-12-28')),'2015-12-28') ORDER BY id_units, id desc) x
GROUP BY id_units;

It is works, but I would like to know if there is another way to improve it

Regard!

Answer 1

This is your query:

SELECT max(swg.id) AS id
FROM swg
WHERE swg.status <> 0 AND
      swg.dateswg <= TIMESTAMPADD(DAY, 6 - WEEKDAY('2015-12-28'), '2015-12-28')
GROUP BY swg.id_units
ORDER BY MAX(swg.id);

This is tricky to optimize, because an index cannot be used for both conditions. You can start with a composite index on all four columns used in the query: swg(dateswg, status, id_units, id)

Answer 2

Of status is only either 0 or 1 , then change to status = 1 so that the Optimizer has one place to look, not two disjoint places. Then have

INDEX(status, dateswg, id_units, id) -- in this order.

That will be pretty good for the WHERE and will be "covering".

Unfortunately both the GROUP BY and the ORDER BY will need a sort; the latter one will be 2000 rows, and probably be done in RAM; I can't tell about the former.