使用復合聚簇索引時，SQL Server是否會跳槽？

Question

請考慮以下復合聚簇索引：

CREATE UNIQUE CLUSTERED INDEX ix_mytable ON mytable(a, b)

顯然，基於B獨立指數將尋求更快B的特定值。

但是，如果沒有使用b上的單獨索引，在我看來，復合索引仍可用於通過遍歷a和離散值的樹來查找具有b的特定值而不是表掃描的元組。對於b本地搜索，跳轉到的 ，等下一個值

這是SQL Server的工作原理嗎？ （例如，如果MSSQL對具有多列的索引使用單個哈希值，則不會這樣。）

它是，並且由於其他原因已經需要復合索引，並且a的離散值的數量足夠小，性能/空間權衡可能擺脫具有b的單獨索引。

（以上UNIQUE和集群約束不是真正需要對於這個例子，但它們將代表B的最快的檢索不涉及一個單獨的索引對於b --the前者提供的快捷方式的每個環路，后者除去查找中的一個間接度）。

Answer 1

不，沒有跳過'a'的集群。 只有在指定了最左邊的列時才能使用索引，否則需要使用完整掃描。

Oracle擁有所謂的“Index Skip Scan”運算符。

Answer 2

USE AdventureWorks2008R2;
-- Source: http://msftdbprodsamples.codeplex.com/releases/view/59211
GO

SET NOCOUNT ON;
GO

CREATE NONCLUSTERED INDEX IX_SalesOrderHeader_OrderDate_#_ShipDate_SubTotal
ON [Sales].[SalesOrderHeader] ([OrderDate])
INCLUDE (ShipDate,SubTotal)
-- WITH(DROP_EXISTING=ON);
GO

-- Test 1
SET STATISTICS IO ON;
SELECT  COUNT(*)
FROM    Sales.SalesOrderHeader h -- Index Seek on IX_SalesOrderHeader_OrderDate_#_ShipDate_SubTotal
WHERE   h.OrderDate BETWEEN '2008-07-01T00:00:00.000' AND '2008-07-15T23:59:59.997';
SET STATISTICS IO OFF;
GO
-- End of Test 1
-- Results:
-- Table 'SalesOrderHeader'. Scan count 1, logical reads 5, physical reads 0

DROP INDEX IX_SalesOrderHeader_OrderDate_#_ShipDate_SubTotal
ON [Sales].[SalesOrderHeader]
GO
CREATE NONCLUSTERED INDEX [IX_SalesOrderHeader_ShipMethodID_OrderDate_#_ShipDate_SubTotal] 
ON Sales.SalesOrderHeader 
(
    ShipMethodID ASC,
    OrderDate ASC
)
INCLUDE (ShipDate,SubTotal);
GO

-- Test 2
SET STATISTICS IO ON;
SELECT  COUNT(*)
FROM    Sales.SalesOrderHeader h -- Index Scan on IX_SalesOrderHeader_ShipMethodID_OrderDate_#_ShipDate_SubTotal
WHERE   h.OrderDate BETWEEN '2008-07-01T00:00:00.000' AND '2008-07-15T23:59:59.997';
SET STATISTICS IO OFF;
GO
-- End of Test 2
-- Results:
-- Table 'SalesOrderHeader'. Scan count 1, logical reads 150, physical reads 0

-- Test 3
SET STATISTICS IO ON;
SELECT  COUNT(*)
FROM    Purchasing.ShipMethod sm
INNER JOIN Sales.SalesOrderHeader h ON h.ShipMethodID=sm.ShipMethodID -- FK elimination + Index Scan on IX_SalesOrderHeader_ShipMethodID_OrderDate_#_ShipDate_SubTotal
WHERE   h.OrderDate BETWEEN '2008-07-01T00:00:00.000' AND '2008-07-15T23:59:59.997';
SET STATISTICS IO OFF;
GO
-- End of Test 3
-- Results:
-- Table 'SalesOrderHeader'. Scan count 1, logical reads 150, physical reads 0

-- Test 4
SET STATISTICS IO ON;
SELECT  MIN(sm.ShipMethodID) AS DummnyCol, -- To prevent FK elimination 
        COUNT(*)
FROM    Purchasing.ShipMethod sm
INNER JOIN Sales.SalesOrderHeader h ON h.ShipMethodID=sm.ShipMethodID -- Index Seek on IX_SalesOrderHeader_ShipMethodID_OrderDate_#_ShipDate_SubTotal
WHERE   h.OrderDate BETWEEN '2008-07-01T00:00:00.000' AND '2008-07-15T23:59:59.997';
SET STATISTICS IO OFF;
GO
-- End of Test 4
-- Results:
-- Table 'SalesOrderHeader'. Scan count 5, logical reads 13, physical reads 0
-- Table 'ShipMethod'. Scan count 1, logical reads 2, physical reads 0

DROP INDEX [IX_SalesOrderHeader_ShipMethodID_OrderDate_#_ShipDate_SubTotal] 
ON Sales.SalesOrderHeader;
GO
SET NOCOUNT OFF;
GO