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SQL字段拆分優化

今天看到一條用函數處理連接的SQL,是群裡某位網友的,SQL語句如下:

SELECT SO_Order.fdate ,
  SO_Order.fsn
FROM so_order
  INNER JOIN so_orderitem ON CHARINDEX(so_Orderitem.fid, SO_Order.fid) >= 1
WHERE so_order.FOrderDate = '2015-09-06'

語句不算復雜,只是執行比較慢,下面是關於這SQL語句的一些信息:

--1.SQL執行203條數據

--2.耗時12秒

--3.so_order表的fid字段是字符串集合,

--由1-2個字符串組成,用','分隔

SELECT COUNT(*)
FROM SO_Order
WHERE so_order.FOrderDate = '2015-09-06'--24
SELECT COUNT(*)
FROM so_Orderitem--414154

  通過分析執行計劃,so_order和so_orderitem走嵌套循環是正確的,查詢的瓶頸是在so_orderitem的索引

掃描上。因此,我們要通過改寫SQL,達到so_orderitem走索引查找的目的。

  將so_order表的fid字段拆分成2個字段,然後union all成一個結果集,再和so_orderitem關聯,即可讓so_orderitem表走索引查找。

改寫的SQL如下:

;WITH x0
   AS ( SELECT fdate ,
      fsn ,
      LEFT(fid, CASE WHEN CHARINDEX(',', fid, 1) = 0 THEN 0
          ELSE CHARINDEX(',', fid, 1) - 1
         END) AS fid
    FROM  so_order
    WHERE FOrderDate = '2015-09-06'
    UNION ALL
    SELECT fdate ,
      fsn ,
      RIGHT(fid, LEN(fid) - CHARINDEX(',', fid, 1)) AS fid
    FROM  so_order
    WHERE FOrderDate = '2015-09-06'
    )
 SELECT SO_Order.fdate ,
   SO_Order.fsn
 FROM x0 so_order
   INNER JOIN so_orderitem ON so_Orderitem.fid = SO_Order.fid

  下面是SQL改寫後,網友的反饋截圖

  改寫後SQL的執行計劃如下:


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