SQL Server自动为主键创建索引，但不为外键创建索引。为外键创建索引。这可能是值得的。

哇，答案五花八门。文档中说:

FOREIGN KEY约束是索引的候选者，因为:

Changes to PRIMARY KEY constraints are checked with FOREIGN KEY constraints in related tables. Foreign key columns are often used in join criteria when the data from related tables is combined in queries by matching the column(s) in the FOREIGN KEY constraint of one table with the primary or unique key column(s) in the other table. An index allows Microsoft® SQL Server™ 2000 to find related data in the foreign key table quickly. However, creating this index is not a requirement. Data from two related tables can be combined even if no PRIMARY KEY or FOREIGN KEY constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria.

因此，很明显(尽管文档有点混乱)，它实际上并没有创建索引。

在PostgeSql中，如果你点击\d tablename，你可以自己检查索引

您将看到，btree索引已经在具有主键和唯一约束的列上自动创建，但没有在具有外键的列上自动创建。

我认为这至少对研究生来说回答了你的问题。

SQL Server自动为主键创建索引，但不为外键创建索引。为外键创建索引。这可能是值得的。

严格来说，外键与索引毫无关系。但是，正如我上面的演讲者指出的，创建一个加速fk查找是有意义的。事实上，在MySQL中，如果你没有在FK声明中指定索引，引擎(InnoDB)会自动为你创建索引。

外键不创建索引。只有备用键约束(UNIQUE)和主键约束才能创建索引。这在Oracle和SQL Server中是正确的。