我有以下查询和php代码：

$sql = "SELECT DISTINCT j.OrderID, DATE_FORMAT(j.StartDate,'%W %e %M %Y %H:%i:%s') AS StartDate, 
                        j.Object, j.Signature,  j.PrintedWithBSN, j.PrintedWithoutBSN,
                        p.ProjectID AS ProjectName, p.City, p.PostalCode, p.StreetName, 
                        c.Name AS CustomerName
        FROM JobOrder j
        INNER JOIN Project p ON p.ProjectID = j.ProjectID
        INNER JOIN Customer c ON c.CustomerID = p.CustomerID
        INNER JOIN OrderEmployer oe ON j.OrderID=oe.OrderID
        INNER JOIN Employer e ON oe.EmployerID=e.EmployerID
        WHERE j.Removed = 0 AND Date(j.StartDate)=:StartDate
        ORDER BY e.Firstname, p.WorkDone, j.Signature, j.StartDate DESC, c.Name";

$query = $database->prepare($sql);

$query->execute(array('StartDate' => $date));";

使用microtimephp函数，我发现 $query->execute 完成大约需要15秒。一年前，查询只需几秒钟就可以执行。我不知道是什么导致了额外的等待时间，是我写的查询不好吗？
-------编辑：我的解释结果：

-------edit2:我的索引是（每个表）：
customer:主键：customerid唯一键：name
雇主：主键：employerid唯一键：用户名
joborder:主键：orderid键：projectid
orderemployer:主键：orderid，employerid键：employerid
项目主键：projectid键：customerid、contactpersonid、contactpersonprojectid