Oracle分布式查询语句的优化

分析：
由于优化器无法判断或获得远端表的统计信息，故原执行计划默认会采取把远程表（无论大小）拉到本地再连接的方式执行，这样如果远程表较大的情况将会比较缓慢，像上述查询耗时在1分钟以上。

原SQL语句：

Select
 Wb.*,
 (Select Wi.Nextarrivedate
    From Mbs7_Oms.Xs_Warearriveinfo@Dc.Moonbasadb.Com Wi
   Where Wi.Warecode = Wb.Warecode) As Nextarrivedate
  From Mbs7_Crm.Wi_Warebase@Dc.Moonbasadb.Com Wb                 ---这里远端表较大
 Inner Join (Select Wa.Stylecode, Max(Wa.Warecode) As Warecode
               From Mbs7_Crm.Wi_Warebase@Dc.Moonbasadb.Com Wa
              Inner Join (Select Stylecode
                           From Dc_Support.Kh_Visitpage Vis
                          Where Vis.Cuscode = :B1
                            And Vis.Addtime >= Trunc(Sysdate - 31)
                            And Vis.Addtime < Trunc(Sysdate - 30)
                            And Rownum <= 5
                          Order By Addtime Desc) Vis
                 On Wa.Stylecode = Vis.Stylecode
              Group By Wa.Stylecode) Wc
    On Wb.Warecode = Wc.Warecode

解决方案：
用以下HINTS方式加上去后，优化器会调整执行计划，把运算端控制在WB,并且远端表MBS7_CRM.WI_WAREBASE表字段的索引(STYLECODE)缺少连接索引，于是在目标端创建以下索引进行优化，优化后COST从7百多下降到20，运行2秒内可返回结果，性能增加不少。

远端表创建索引：

create index mbs7_crm.ix_WI_WAREBASE_STYLECODE on mbs7_crm.WI_WAREBASE(STYLECODE)

调优后的SQL：

Select /*+DRIVING_SITE(WB)*/
 Wb.*,
 (Select Wi.Nextarrivedate
    From Mbs7_Oms.Xs_Warearriveinfo@Dc.Moonbasadb.Com Wi
   Where Wi.Warecode = Wb.Warecode) As Nextarrivedate
  From Mbs7_Crm.Wi_Warebase@Dc.Moonbasadb.Com Wb                 ---远端表较大
 Inner Join (Select Wa.Stylecode, Max(Wa.Warecode) As Warecode
               From Mbs7_Crm.Wi_Warebase@Dc.Moonbasadb.Com Wa
              Inner Join (Select Stylecode
                           From Dc_Support.Kh_Visitpage Vis
                          Where Vis.Cuscode = :B1
                            And Vis.Addtime >= Trunc(Sysdate - 31)
                            And Vis.Addtime < Trunc(Sysdate - 30)
                            And Rownum <= 5
                          Order By Addtime Desc) Vis
                 On Wa.Stylecode = Vis.Stylecode
              Group By Wa.Stylecode) Wc
    On Wb.Warecode = Wc.Warecode