Postgresql 中文操作指南
11.6. Unique Indexes #
索引还可用于强制列值的唯一性或多个列的组合值的唯一性。
Indexes can also be used to enforce uniqueness of a column’s value, or the uniqueness of the combined values of more than one column.
CREATE UNIQUE INDEX name ON table (column [, ...]) [ NULLS [ NOT ] DISTINCT ];目前，只有 B 树索引可声明为唯一。
Currently, only B-tree indexes can be declared unique.
当索引声明为唯一时，不允许具有相等索引值的多个表行。默认情况下，唯一列中的空值不被视为相等，允许列中有多个空值。NULLS NOT DISTINCT 选项会修改此项并使索引将空值视为相等。多列唯一索引将仅拒绝在多行中所有索引列都相等的情况。
When an index is declared unique, multiple table rows with equal indexed values are not allowed. By default, null values in a unique column are not considered equal, allowing multiple nulls in the column. The NULLS NOT DISTINCT option modifies this and causes the index to treat nulls as equal. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows.
当对表定义唯一约束或主键时，PostgreSQL 会自动创建一个唯一索引。索引会涵盖构成主键或唯一约束的列（如果合适，则涵盖多列索引），它是强制执行约束的机制。
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. The index covers the columns that make up the primary key or unique constraint (a multicolumn index, if appropriate), and is the mechanism that enforces the constraint.