创建constraint时的选项：not deferrable和deferrable

使用语法：not deferrable | deferrable [ initially { immediate | deferred } ]

not deferrable 是不可延迟，并且约束也无法更改为可延迟状态。约束会在每一句sql statement 之后都进行 check，不符合则 roll back

deferrable 是可延迟状态，在 deferrable 状态时又有2个选项，创建时缺省是initially immediate，即约束会在每一句sql statement 之后都进行 check，效果等同于not deferrable，但是可以修改延迟状态。

initially deferred 约束会在整个事务进行commit 时check，如果不符合则roll back

在deferrable状态下，可以通过set constraints修改 immediate或者deferred

Synopsis

SET CONSTRAINTS { ALL | constraint [,... ] }
                { DEFERRED | IMMEDIATE }

Parameters

ALL
The keyword indicating that the mode you are specifying should be applied to all constraints within the current transaction.

constraint
The name of a specific constraint of which you wish to set the mode.

DEFERRED
The keyword indicating that constraints (or a specific constraint) shouldn't be checked until the transaction reaches a COMMIT.

IMMEDIATE
The keyword indicating that constraints (or a specific constraint) should be checked at the end of each statement within a transaction.

Results

SET CONSTRAINTS
The message returned when a constraint mode is set successfully.

ERROR: Constraint 'constraint' does not exist
The error returned if you attempt to change the mode of a constraint that does not exist.

Description

Use the SET CONSTRAINTS command to set the constraint mode for all constraints or for a single constraint within the current transaction block. You can choose to set the constraint mode to either IMMEDIATE or DEFERRED.
Use of IMMEDIATE mode will force the checking of all constraints at the end of each statement within the transaction. In DEFERRED mode, constraints are not checked until a COMMIT command
is issued.

Note: PostgreSQL 7.1.x (the most current version as of the writing of this book) only supports the use of these modes with foreign key constraints, as both check and unique constraints are set to a constraint mode that is not affected by
this command.

Example

The following example sets the constraint evaluation mode to IMMEDIATE for all constraints within the transaction:

SET CONSTRAINTS ALL IMMEDIATE;