Index unique scan is one of the most efficient ways of accessing data. This access
method is used for returning the data from B-tree indexes. The optimizer chooses a
unique scan when all columns of a unique (B-tree) index are specified with equality
conditions.

索引唯一扫描
当一个唯一索引的某些与查询体条件表达式相等时，优化器将选择使用索引唯一扫描