是这样的:
Postgresql 认为null都是不相等的,多栏键值的唯一性只有全部是非NULL的情况下才可进行比较。
Null values are not considered equal. A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows.
好在postgresql有表达式索引:
create unique index xxx on user ((coalesce(pid,-1)||code))