Question

我有一个非常复杂的SQL视图。我的问题是，当我想要报告时，它非常慢并且需要大量内存。观点的作用：它查询总值并将其乘以一个因子显然这是因为它包含了很多子查询......（如果算上9则）

查询在具有数百万个数据集的系统上运行。

我的问题。哪些点可以优化呢？如果我制作准备好的程序或者为子查询创建sql视图会更快吗？

SELECT 
    CASE BID WHEN 1 THEN 
        b.TOTAL 
    ELSE 
        (b.TOTAL * - 1) 
    END 
    * 
    CASE WHEN
        (SELECT 
            (CAST(DATEDIFF(day, 
                ISNULL
                ((SELECT dbo.TableA.TimestampFrom + 1 AS start FROM dbo.TableB INNER JOIN
                    dbo.TableA ON dbo.TableB.SID = dbo.TableA.SID
                    WHERE   (dbo.TableB.PID = dbo.TableC.PID) AND
                        (dbo.TableB.MString = 'S' OR dbo.TableB.MString = 'ST') AND
                        (dbo.TableB.TimestampCreated <
                            (SELECT TimestampCreated
                                FROM dbo.TableB AS TableB_2
                            WHERE (RID = dbo.TableC.RID)))
                        AND
                            (dbo.TableB.VPFrom =
                                    (SELECT VPFrom FROM dbo.TableB AS TableB_1
                                            WHERE (RID = dbo.TableC.RID))
                            )), VPFrom), VPTo)
                            AS Float) + 1)
            / 
            (CAST(DATEDIFF(day, start, Vertragsende) AS Float) + 1) AS Faktor

            FROM dbo.TableC
                WHERE (LaufNr = b.LaufNr))
                    > 1 
        THEN 
            1 
        ELSE
            (SELECT (CAST(DATEDIFF(day, 
                ISNULL
                    ((SELECT dbo.TableA.TimestampFrom + 1 AS start FROM dbo.TableB INNER JOIN
                            dbo.TableA ON dbo.TableB.SID =  dbo.TableA.SID
                            WHERE (dbo.TableB.PID = dbo.TableC.PID) AND
                                (dbo.TableB.MString = 'S' OR dbo.TableB.MString = 'ST') AND
                                (dbo.TableB.TimestampCreated <
                                    (SELECT TimestampCreated FROM dbo.TableB AS TableB_2
                                     WHERE (RID = dbo.TableC.RID)))
                                AND (dbo.TableB.VPFrom =
                                    (SELECT VPFrom FROM dbo.TableB AS TableB_1
                                    WHERE (RID = dbo.TableC.RID)))
                        ), VPFrom),VPTo) AS Float) + 1)
                / (CAST(DATEDIFF(day, start, Vertragsende) AS Float) + 1) AS Faktor 
                FROM dbo.TableC
                WHERE (LaufNr = b.LaufNr)) 
        END AS GrossPremium
    ,RID, PID, ONR, OPosLaufNr, LaufNr, bdate
FROM dbo.TableC AS b

Answer 1

索引的经验法则是

首先，阅读执行计划，

然后考虑添加一个索引

WHERE子句中使用的每一列
在JOIN条件中使用的每一列，
ORDER BY中使用的每一列。

在某些情况下，SELECT语句中的一个多列索引比几个单列索引更快。

然后，确保您的WHERE子句为sargable。

最后，尝试将一些子查询转换为视图以进行测试是有意义的。我的猜测是问题的根本原因是你根据几个CASE语句的返回值执行不同的子查询。

如何使用子查询优化复杂的SQL查询/视图

1 个答案: