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INTEGER PRIMARY KEY 列放在一边，UNIQUE 和 PRIMARY KEY约束是通过在数据库中创建索引来实现的(在与CREATE UNIQUE INDEX"语句相同的方式).这样一个索引像数据库中的任何其他索引一样用于优化查询.因此，通常没有优势(但显着的开销)在一组已经存在的列上创建索引共同受 UNIQUE 或 PRIMARY KEY 约束.

When an integer column is marked as a primary key in an SQLite table, should an index be explicitly created for it as well? SQLite does not appear to automatically create an index for a primary key column, but perhaps it indexes it anyway, given its purpose? (I will be searching on that column all the time).

Would the situation be any different for a string primary key?

It does it for you.

INTEGER PRIMARY KEY columns aside, both UNIQUE and PRIMARY KEY constraints are implemented by creating an index in the database (in the same way as a "CREATE UNIQUE INDEX" statement would). Such an index is used like any other index in the database to optimize queries. As a result, there often no advantage (but significant overhead) in creating an index on a set of columns that are already collectively subject to a UNIQUE or PRIMARY KEY constraint.

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