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SQL中的表扫描和索引扫描之间的区别是什么？详细行使它在那边？
办理要领
表扫描意味着遍历全部表行.
索引扫描意味着遍历全部索引项,当项索引满意搜刮前提时,通过索引检索表行.
Usualy索引扫描比表扫描自制,由于索引比表更平展.
关于这个题目,他们有许多书目.样品：
>微软：Which is Faster: Index Access or Table Scan?：
Index access is an access method in which SQL Server uses an existing
index to read and write data pages. Because index access significantly
reduces the number of I/O read operations,it often outperforms a
table scan.
> Oracle：The Query Optimizer
In this method,a row is retrieved by traversing the index,using the
indexed column values specified by the statement. An index scan
retrieves data from an index based on the value of one or more columns
in the index. To perform an index scan,Oracle searches the index for
the indexed column values accessed by the statement. If the statement
accesses only columns of the index,then Oracle reads the indexed
column values directly from the index,rather than from the table.
> MySql：How to Avoid Table Scans （编辑：湖南网）
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