【发布时间】：2018-12-31 04:47:00
【问题描述】：
我继承了 Azure 中托管的 MS Sql 数据库。 寻找性能改进，我一直在阅读很多关于索引和覆盖索引的内容。 （也许这是我找到的最完整的阅读：https://www.red-gate.com/simple-talk/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/）
但仍有一个疑问......
因此，例如，对于下面的计费表（大约有 800 万行），我发现查询的 where 子句中使用最多的字段是（是否在连接内）：
PAYMENT_DATE, DUE_DATE, CUSTOMER_ID, DELAY_DAYS, AMOUNT
.
CREATE TABLE [dbo].[BILLING](
[ID] [int] IDENTITY(1,1) NOT NULL,
[CHANGED_DATE] [datetime] NULL,
[INCLUDED_DATE] [datetime] NULL,
[CHANGED_USER_ID] [int] NULL,
[INCLUDED_USER_ID] [int] NULL,
[BILL_CODE] [varchar](255) NOT NULL,
[PAYMENT_DATE] [datetime] NULL,
[DUE_DATE] [datetime] NOT NULL,
[AMOUNT] [float] NOT NULL,
[AMOUNT_PAYED] [float] NULL,
[CUSTOMER_ID] [int] NOT NULL,
[OUR_NUMBER] [varchar](200) NULL,
[TYPE] [varchar](250) NULL,
[BANK_ID] [int] NULL,
[ISSUE_DATE] [datetime] NULL,
[STATE] [varchar](20) NULL,
[DUNNING_STATE_ID] [int] NULL,
[OPEN_VALUE] [float] NULL,
[ACCREDIT_VALUE] [float] NULL,
[LOWER_VALUE] [float] NULL,
[DISCCOUNT_VALUE] [float] NULL,
[INTEREST_VALUE] [float] NULL,
[FINE_VALUE] [float] NULL,
[RECEIVED_AMOUNT] [float] NULL,
[DELAY_DAYS] [int] NULL,
[BRANCH_ID] [int] NULL,
[FIELD1] [varchar](250) NULL,
[FIELD2] [varchar](250) NULL,
[FIELD3] [varchar](250) NULL,
[FIELD4] [varchar](250) NULL,
[FIELD5] [varchar](250) NULL,
[OBS1] [varchar](250) NULL,
[OBS2] [varchar](250) NULL,
[OBS3] [varchar](250) NULL,
[INTEREST_RATE] [float] NULL,
[INTEREST_CALC] [float] NULL,
[AGREEMENT_STATE] [varchar](20) NULL,
[AGREEMENT_ID] [int] NULL,
PRIMARY KEY CLUSTERED
(
[ID] ASC
)
此外，用于优化的目标查询对 select 子句进行计算：
AMOUNT, DELAY_DAYS, COUNT(ID)。
例如：
SELECT
T.CUSTOMER_ID AS CUSTOMER_ID
, COUNT(T.ID) AS NUM_BILLS
, COUNT(
CASE
WHEN T.DELAY_DAYS <= 0 THEN 1
ELSE NULL
END
) AS DEPOSITS
, COUNT(
CASE
WHEN T.DELAY_DAYS > 0 THEN 1
ELSE NULL
END
) AS DEFAULTED
, COUNT(
CASE
WHEN T.DELAY_DAYS BETWEEN 30 AND 60 THEN 1
ELSE NULL
END
) AS DEFAULTED_30
, COUNT(
CASE
WHEN T.DELAY_DAYS BETWEEN 60 AND 90 THEN 1
ELSE NULL
END
) AS DEFAULTED_60
, COUNT(
CASE
WHEN T.DELAY_DAYS > 90 THEN 1
ELSE NULL
END
) AS DEFAULTED_90
, MAX(T.DELAY_DAYS) AS MAX_DEFAULTED_TIME
, SUM(
CASE
WHEN T.DELAY_DAYS > 0 THEN T.DELAY_DAYS
ELSE 0
END
) AS SUM_DEFAULTED_TIME
, SUM(T.AMOUNT) AS AMOUNT
, SUM(
CASE
WHEN T.DELAY_DAYS > 0 THEN T.AMOUNT
ELSE 0
END
) AS DEFAULTED_AMOUNT
FROM BILLING T
WHERE
T.DUE_DATE < GETDATE()
AND T.AMOUNT > 0
GROUP BY
T.CUSTOMER_ID
因此，在我看来，以下索引可以解决我的所有问题：
CREATE NONCLUSTERED INDEX [ix_Titulo_main_fields] ON [dbo].[BILLING]
(
[PAYMENT_DATE] ASC,
[DUE_DATE] DESC,
[AMOUNT] ASC,
[CUSTOMER_ID] ASC,
[STATE] ASC,
[DELAY_DAYS] ASC,
[BRANCH_ID] ASC,
[AGREEMENT_ID] ASC
)
INCLUDE ( [BILLING_CODE],
[AGREEMENT_STATE],
)
GO;
相比之下，当我在 Management Studio 上询问查询计划时，SQL Server 不使用此索引并建议我创建一个新索引：
CREATE NONCLUSTERED INDEX [ix_billing_due_date_amount] ON [dbo].[billing]
(
[due_date] ASC,
[amount] ASC
)
INCLUDE ( [customer_id],
[delay_days])
GO
所以，疑问是：
覆盖索引是否需要正是 WHERE 子句搜索的内容？
如果是真的，覆盖索引如何满足多个查询？
否则为什么以前的索引不满足查询？
我真的不知道我错过了什么......
提前致谢！
【问题讨论】：
标签： sql-server indexing covering-index