... 组合索引（Composite Indexes / concatenated index） 索引和键（Indexes and Keys） 索引和 NULL（Indexes and Nulls） ...
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Provides access to common queries based on primary keys, foreign keys, and indexes.
为基于主键、外键和索引的通用查询提供访问
Indexes on the foreign keys referencing tables with few rows have low selectivity, and the utility of requiring the index has always been questionable.
外键上的索引所引用的表行数较少，选择性较少，需要索引的实用程序一直遭受质疑。
The reason: Normalization requires more tables, table joins, keys and indexes and thus more internal database operations for implement queries.
其原因是：规范化需要更多的表，连接等操作，主键、索引，从而需要更多的内部操作。
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