initially immediate 与 initially deferred

initially immediate 初始化立即执行.

initially deferred 初始化延迟执行.

表约束定义：

      [ CONSTRAINT name ]          { PRIMARY KEY constraint |            FOREIGN KEY constraint |           UNIQUE constraint |            CHECK constraint }           [ {INITIALLY DEFERRED | INITIALLY IMMEDIATE} ]           [ [ NOT ] DEFERRABLE ]

列约束定义：

     [ CONSTRAINT name ]          { NOT NULL constraint |           PRIMARY KEY constraint |            FOREIGN KEY constraint |            UNIQUE constraint |            CHECK constraint }             [ {INITIALLY DEFERRED | INITIALLY IMMEDIATE} ]           [ [ NOT ] DEFERRABLE ]

一个 CONSTRAINT （约束）定义可以包含一个任意顺序的延迟属性的子句和/或一个初始化约束式子句．

• NOT DEFERRABLE

• 意味着每条 SQL 语句执行后都必须校验是否违反约束规则．

• DEFERRABLE

• 意味着约束的校验可以推迟到晚些时候，但不晚于当前事务的结束．

事务开始时，每个约束会为每个约束的模式设置一个初始值．

• INITIALLY IMMEDIATE

• 意味着，当事务开始时，每个 SQL 语句执行后都必须检查是否违反了约束规则．

• INITIALLY DEFERRED

• 意味着，当事务开始后，对约束的检查可以推迟到晚些时候，但不晚于当前事务的结束．