拆分字段优化SQL

　

拆分字段优化SQL

今天看到一条用函数处理连接的SQL，是群里某位网友的，SQL语句如下：

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‘

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‘

　　语句不算复杂，只是执行比较慢，下面是关于这SQL语句的一些信息：

--1.SQL执行203条数据
--2.耗时12秒
--3.so_order表的fid字段是字符串集合，
--由1-2个字符串组成,用‘,‘分隔
SELECT  COUNT(*)
FROM    SO_Order
WHERE   so_order.FOrderDate = ‘2015-09-06‘--24

SELECT  COUNT(*)
FROM    so_Orderitem--414154

　　

　　通过分析执行计划，so_order和so_orderitem走嵌套循环是正确的，查询的瓶颈是在so_orderitem的索引

扫描上。因此，我们要通过改写SQL，达到so_orderitem走索引查找的目的。

　　将so_order表的fid字段拆分成2个字段，然后union all成一个结果集，再和so_orderitem关联，即可让

so_orderitem表走索引查找。改写的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

;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改写后，网友的反馈截图

　　改写后SQL的执行计划如下：