这是SQL Server 2008的新特性，它使我们可以向索引增加WHERE子句，这样就可以将索引聚焦到大多数重要的行上。
New for SQL Server 2008, permit you to add a WHERE clause to an index, thus focusing the index on the most important rows.
众多恒行最紧密地聚焦在银河系中心，因此它们创造出赋予生命的重元素在那里也最丰富。
Stars congregate most tightly at the galaxy's center, so the life-giving heavy elements they create are most plentiful there.
从现在开始，我们将省略纯诊断性代码行，以便聚焦于直接执行程序功能的那些代码行。
From this point forward, lines that are purely diagnostic are omitted so that we can concentrate on just those that contribute directly to performing program functions.
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