Indexes improve query performance by avoiding a full table scan.
索引能够避免全表扫描，从而能够提高查询性能。
The preceding query requires MySQL to perform a full table scan, the slowest type of query for a large table.
上面的那个查询会执行一个全表扫描，对于一个大表来说这是最慢的一种查询方式。
The predicates in the query in Listing 7 are weakly selective so that a full table scan of the PDBXML table is required.
清单7中查询的谓词选择性较弱，因此需要对pdbxml表进行全表扫描。
Most often, you want only a small subset of the data in the table, so a full table scan wastes a lot of disk I/O and therefore time.
通常来说，您可能只希望获得表中数据的一个子集，因此全表扫描会浪费大量的磁盘I/O，因此也就会浪费大量时间。
If there is no index on ucname, the DBMS still does a full table scan, but it need not do any further processing on the data to determine whether or not it satisfies the query.
如果没有为ucname创建索引，DBMS仍然执行全表扫描，但是不会对数据进行进一步处理来判断其是否满足查询。
If you assume that there is an index on COL1 in the above example, if COL1 is type-cast instead of the RHS, then the index will be bypassed and the query will perform a full table scan.
假设上面示例中的COL1列上有索引，如果对COL1而不是RHS执行类型转换，那么会绕过索引，查询会执行全表扫描。
If you assume that there is an index on COL1 in the above example, if COL1 is type-cast instead of the RHS, then the index will be bypassed and the query will perform a full table scan.
假设上面示例中的COL1列上有索引，如果对COL1而不是RHS执行类型转换，那么会绕过索引，查询会执行全表扫描。
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