使用复合聚簇索引时，SQL Server是否会跳槽？

Question

请考虑以下复合聚簇索引：

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

显然，基于B独立指数将寻求更快B的特定值。

但是，如果没有使用b上的单独索引，在我看来，复合索引仍可用于通过遍历a和离散值的树来查找具有b的特定值而不是表扫描的元组。对于b本地搜索，跳转到的 ，等下一个值

这是SQL Server的工作原理吗？ （例如，如果MSSQL对具有多列的索引使用单个哈希值，则不会这样。）

它是，并且由于其他原因已经需要复合索引，并且a的离散值的数量足够小，性能/空间权衡可能摆脱具有b的单独索引。

（以上UNIQUE和集群约束不是真正需要对于这个例子，但它们将代表B的最快的检索不涉及一个单独的索引对于b --the前者提供的快捷方式的每个环路，后者除去查找中的一个间接度）。

Answer 1

不，没有跳过'a'的集群。 只有在指定了最左边的列时才能使用索引，否则需要使用完整扫描。

Oracle拥有所谓的“Index Skip Scan”运算符。

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