今天收到一個需求，要改寫一個報表的邏輯，當改完之後，再次執行，發現執行超時。

因為特殊原因，無法訪問客戶的伺服器，沒辦法檢視sql的執行計劃、沒辦法知道表中的索引情況，所以，嘗試從語句的改寫上來優化。

一、原始語句如下：

select isnull(vv.customer_id,v.customer_id) as customer_id,isnull(vv.business_date,replace(v.business_date,'-','')) as business_date,v.prod_id,v.sales,vv.visit_count,v.all_sales
from 
(
  SELECT a.customer_id,max(month)+'-01' as business_date,a.PROD_ID,SUM(CAST(VALUE AS NUMERIC(38,3))) sales,sum(SUM(CAST(VALUE AS NUMERIC(38,3)))) over(partition by a.customer_id) as all_sales
							
  FROM  TB_IMPORT_SALES a 
  WHERE  a.customer_id IS NOT NULL
      AND a.PROD_ID IS NOT NULL
			and a.month='2016-11'
  GROUP BY a.customer_id,a.PROD_ID
)v
full join
(
  SELECT customer_id,max(a.business_date) as business_date,COUNT(*) AS VISIT_COUNT 
	FROM TB_CALL_STORE a WITH(NOLOCK)
	inner join TB_TIME d
	on a.business_date = d.t_date 
	where d.section ='2016-11'
	GROUP BY customer_id
)vv
on v.customer_id = vv.customer_id

原來是left join，雖然查詢比較慢，但是2分鐘能查出來，現在按照業務要求，需要看到所有資料，所以改成了full join，改了之後5分鐘都查不出結果。

二、改寫後的程式碼

select v.customer_id,replace(max(v.business_date),'') as business_date,max(v.sales_volume) sales_volume,max(v.visit_count) visit_count,max(v.all_sales_volume) all_sales_volume
from 
(
  SELECT a.customer_id,max(biz_month)+'-01' as business_date,SUM(CAST(VALUE1 AS NUMERIC(38,8))) sales_volume,sum(SUM(CAST(VALUE1 AS NUMERIC(38,8)))) over(partition by a.customer_id) as all_sales_volume,null as visit_count
							
  FROM  TB_IMPORT_SALES a 
  WHERE  a.customer_id IS NOT NULL
      AND a.PROD_ID IS NOT NULL
			and a.month='2016-11'
  GROUP BY a.customer_id,a.PROD_ID
  union all
 
  SELECT customer_id,p.prod_id,null,COUNT(*) AS VISIT_COUNT 
	FROM TB_CALL_STORE a WITH(NOLOCK)
	cross apply
	(
		select top 1 prod_id from TB_PRODUCT with(nolock)
	)p
	inner join TB_TIME d
	on a.business_date = d.t_date 
	where d.section ='2016-11'
	GROUP BY customer_id,p.prod_id
)v
group by v.customer_id,v.prod_id

由於程式碼本身比較簡單，沒辦法再進一步簡化，而由於連線不了伺服器，其他的方法也用不上，甚至沒辦法分析到底是什麼導致執行這麼慢。

想了想，full join 本質上就是 2次left join+union ，無非就是合併資料，於是嘗試一下用union all來直接合並資料，現在改成unoin all最後，就不需要full join。

但是考慮到第2段程式碼中並沒有prod_id這個欄位，所以這裡在第2段程式碼加上了cross apply隨便取出一個產品的id，這樣就有prod_id這個欄位，可以合併了。

修改之後，果然速度降到了10多秒。

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