我在两个表上有一个简单的连接，大约需要50秒来执行，我不太清楚为什么？
代码如下：

SELECT

  LEFT(t1.group, 3) AS `Group`,
  max(t1.`Z-indexnummer`) As 'ZI-Nummer',
  count(t1.`Z-indexnummer`) As 'PacksDispensed',
  t2. `Artikelomschrijving: inkoophoeveelheid (aantal)`as 'Packsize',
  t2.`Prijs: apotheekinkoop (ex. BTW)` As 'Prijs EX BTW',
  Afleverapotheek, Zorgverzekeraar, Basisproduct, Etiketnaam,
  t2.`Gm.: productnaam: PRK=prescriptie (code)`, t2.`Naam: productverantwoordelijke`,
  t2.`Inkoopkanaal (code)`,
  t3.mnvddd
FROM My_Data as t1
INNER JOIN ZINummer as t2 ON t1.`Z-indexnummer` = t2.`Artikelnummer: ZI-nummer`
INNER JOIN Idea as t3 ON t1.`Z-indexnummer` = t3.atkode
WHERE 

t1.`VPV-Weekaflevering` = '0, Gewone Levering'

  group by t1.`Z-indexnummer`
  order by PacksDispensed desc;

有人有什么想法吗？非常感谢你的帮助。
谢谢