我对DB的接触有限,只是作为应用程序程序员使用过DB。我想知道关于聚集和非聚集索引。
我在谷歌上搜索了一下,我发现:
A clustered index is a special type of index that reorders the way
records in the table are physically
stored. Therefore table can have only
one clustered index. The leaf nodes
of a clustered index contain the data
pages. A nonclustered index is a
special type of index in which the
logical order of the index does not
match the physical stored order of
the rows on disk. The leaf node of a
nonclustered index does not consist of
the data pages. Instead, the leaf
nodes contain index rows.
我在SO中发现的是聚集索引和非聚集索引之间的区别是什么?
有人能用通俗易懂的语言解释一下吗?
我知道这是一个非常古老的问题,但我想我可以提供一个类比来帮助说明上面的答案。
聚集索引
If you walk into a public library, you will find that the books are all arranged in a particular order (most likely the Dewey Decimal System, or DDS). This corresponds to the "clustered index" of the books. If the DDS# for the book you want was 005.7565 F736s, you would start by locating the row of bookshelves that is labeled 001-099 or something like that. (This endcap sign at the end of the stack corresponds to an "intermediate node" in the index.) Eventually you would drill down to the specific shelf labelled 005.7450 - 005.7600, then you would scan until you found the book with the specified DDS#, and at that point you have found your book.
非聚簇索引
But if you didn't come into the library with the DDS# of your book memorized, then you would need a second index to assist you. In the olden days you would find at the front of the library a wonderful bureau of drawers known as the "Card Catalog". In it were thousands of 3x5 cards -- one for each book, sorted in alphabetical order (by title, perhaps). This corresponds to the "non-clustered index". These card catalogs were organized in a hierarchical structure, so that each drawer would be labeled with the range of cards it contained (Ka - Kl, for example; i.e., the "intermediate node"). Once again, you would drill in until you found your book, but in this case, once you have found it (i.e, the "leaf node"), you don't have the book itself, but just a card with an index number (the DDS#) with which you could find the actual book in the clustered index.
当然,没有什么能阻止图书管理员复印所有的卡片,并将它们按不同的顺序分类在一个单独的卡片目录中。(通常至少有两个这样的目录:一个按作者姓名排序,另一个按标题排序。)原则上,您可以拥有任意数量的这些“非聚集”索引。
聚集索引
聚集索引根据表或视图中的键值对数据行进行排序和存储。这些是包含在索引定义中的列。每个表只能有一个聚集索引,因为数据行本身只能按一种顺序排序。
只有当表中包含聚集索引时,表中的数据行才会按排序顺序存储。当一个表具有聚集索引时,这个表称为聚集表。如果表没有聚集索引,则其数据行存储在称为堆的无序结构中。
非聚集
Nonclustered indexes have a structure separate from the data rows. A nonclustered index contains the nonclustered index key values and each key value entry has a pointer to the data row that contains the key value.
The pointer from an index row in a nonclustered index to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a heap or a clustered table. For a heap, a row locator is a pointer to the row. For a clustered table, the row locator is the clustered index key.
可以将非键列添加到非聚集索引的叶级,以绕过现有的索引键限制,并执行完全覆盖的索引查询。有关更多信息,请参见创建包含列的索引。有关索引键限制的详细信息,请参见SQL Server最大容量规格。
参考:https://learn.microsoft.com/en-us/sql/relational-databases/indexes/clustered-and-nonclustered-indexes-described