逻辑读取数量随着一个额外的内部连接而爆炸

Posted 2021-05-03

我有两张看起来几乎相同的桌子。当我从其中任何一个中选择时，我的逻辑读取大约为8.000-10.000次读取。但当我内心加入它们时，我得到了大约380.000个逻辑读取。

我使用MS SQL 2012。

我遇到问题的查询如下所示：

SELECT ac.ID AS AccountID
    ,ab.Balance
    ,abc.BalanceInAccountCurrency
FROM dbo.Dates d
INNER JOIN dbo.Accounts ac ON d.[Date] BETWEEN ac.CreationDate AND ac.ClosureDate
INNER JOIN dbo.AccountBalances ab ON ab.AccountID  = ac.ID AND d.[Date] BETWEEN ab.CreationDate  AND ab.ClosureDate
INNER JOIN dbo.AccountBalancesInAccountCurrency abc ON abc.AccountID = ac.ID AND d.[Date] BETWEEN abc.CreationDate AND abc.ClosureDate
WHERE d.[Date] = DATEFROMPARTS(2017,06,20);

当我只加入AccountBalances时：

SELECT ac.ID AS AccountID
      ,ab.Balance
FROM dbo.Dates d
INNER JOIN dbo.Accounts ac ON d.[Date] BETWEEN ac.CreationDate AND ac.ClosureDate
INNER JOIN dbo.AccountBalances ab ON ab.AccountID  = ac.ID AND d.[Date] BETWEEN ab.CreationDate  AND ab.ClosureDate
WHERE d.[Date] = DATEFROMPARTS(2017,06,20);

当我只加入AccountBalancesInAccountCurrency时，我得到了类似的结果。

我的主键/聚簇索引在两个表上都是这样的：

ALTER TABLE [dbo].[AccountBalances] ADD  CONSTRAINT [PK_AccountBalances] PRIMARY KEY CLUSTERED 
(
    ClosureDate DESC,
    CreationDate DESC,
    AccountID ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

我究竟做错了什么？

执行计划

Both Tables

AccountBalancesInAccountCurrency Only

AccountBalances Only

额外信息

查询结果为170460行，总逻辑读取为376.000。但如果我在查询中使用“top 170000”，那么逻辑读取总数仅为44.000。统计IO看起来像这样：

And the execution plan looks like this

答案

我找到了解决问题的方法。

1. AccountBalances包含银行会计货币的余额，即DKK，AccountBalancesInAccountCurrency包含帐户货币和DKK的余额。大多数货币都是DKK，存在于两个表中。因此，我从AccountBalancesInAccountCurrency中删除了所有DKK余额，并对该表进行了左连接。

2.我还更改了AccountBalancesInAccountCurrency上的主键：

ALTER TABLE [dbo].[AccountBalances] ADD  CONSTRAINT [PK_AccountBalances] 
PRIMARY KEY CLUSTERED 
(
    ClosureDate DESC,
    CreationDate DESC,
    AccountID ASC
)

至：

ALTER TABLE [dbo].[AccountBalancesInAccountCurrency] ADD  CONSTRAINT 
[PK_AccountBalancesInAccountCurrency] PRIMARY KEY CLUSTERED 
(
    [AccountID] ASC,
    [ClosureDate] DESC
)

现在我的IO看起来像这样：

and my execution plan like this。

感谢所有引导我前进的评论。

节日快乐！