一个子查询中的sql优化

  select a.employeeid,
         (select listagg(g.Email, ';') within group(order by e.CustomID)
            from LCInsured e, LIInsContact f, LIBranchEmailSet g
           where e.CustomID = f.CustomID
             and e.EmployeeID = f.EmployeeID
             and f.ContactNo = g.ContactNo
             and f.Relation = '01'
             and g.IsValid = '01'
             AND g.emailstatus = '01'
             and e.Relationship = '01'
          --A  and e.employeeid = a.employeeid
          --A  and e.grpcontno = a.grpcontno
          --B and e.insuredid =  (select insuredid from lcinsured where grpcontno = a.grpcontno and employeeid = a.employeeid and relationship='01')
          )
    FROM (select *
            from (select a.*,
                         ROW_NUMBER() OVER(partition by a.customid order by a.expirydate desc) rn
                    from lcinsured a
                   where 1 = 1
                     and a.effectivedate <= a.expirydate
                     and a.grpcontno = 'MSH202006157963')
           where rn = 1) a
    left join LCGrpContRenewMiddleinsured b
      on b.customid = a.customid
     and b.employeeid = a.employeeid
    join lcgrpplan c
      on a.grpcontno = c.grpcontno
     and a.grpplancode = c.grpplancode
   order by a.insuredid

我不知道为什么. 反正结果是
A 11秒
B 1秒
虽然优化了 但不知道原因. 这个sql是取被保人的主被保人的邮箱地址.