If the table in your query are pretty small (usually fewer than 1,000 rows) or clustered tables, and your query is not selective at all, it would be beneficial to use a table scan.
如果查询中的表极其小(通常少于1,000行)或者是群集表，而且查询根本没有选择性的话，那么使用表扫描会更好一些。
By contrast, an index that is not selective is not as useful.
与之相比，选择性低的索引就不那么有用了。
Bubble Chambers are not selective because they cannot be triggered.
气泡室由于不能触发而没有选择性。
Bubble Chambers are not selective because they cannot be triggered.
气泡室由于不能触发而没有选择性。
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