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sql優(yōu)化實戰(zhàn) 把full join改為left join +union all(從5分鐘降為10秒)

更新時間：2020年09月19日 16:36:31 作者：陽泉酒家小當(dāng)家

因為特殊原因，無法訪問客戶的服務(wù)器，沒辦法查看sql的執(zhí)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(yōu)化

今天收到一個需求，要改寫一個報表的邏輯，當(dāng)改完之后，再次運行，發(fā)現(xiàn)運行超時。

因為特殊原因，無法訪問客戶的服務(wù)器，沒辦法查看sql的執(zhí)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(yōu)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(xiàn)在按照業(yè)務(wù)要求，需要看到所有數(shù)據(jù)，所以改成了full join，改了之后5分鐘都查不出結(jié)果。

二、改寫后的代碼

select v.customer_id,
		replace(max(v.business_date),'-','') as business_date,
		v.prod_id,
		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,
      a.PROD_ID ,
      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, 
	    max(a.business_date) as business_date,
		  p.prod_id,
		  null,
		  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

由于代碼本身比較簡單，沒辦法再進(jìn)一步簡化，而由于連接不了服務(wù)器，其他的方法也用不上，甚至沒辦法分析到底是什么導(dǎo)致運行這么慢。

想了想，full join 本質(zhì)上就是 2次left join+union ，無非就是合并數(shù)據(jù)，于是嘗試一下用union all來直接合并數(shù)據(jù)，現(xiàn)在改成unoin all最后，就不需要full join。

但是考慮到第2段代碼中并沒有prod_id這個字段，所以這里在第2段代碼加上了cross apply隨便取出一個產(chǎn)品的id，這樣就有prod_id這個字段，可以合并了。

修改之后，果然速度降到了10多秒。

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