NAME

SET CONSTRAINTS - 設置當前事務的約束模式

SYNOPSIS

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

DESCRIPTION 描述

SET CONSTRAINTS 設置當前事務里的約束運算的特性。 在 IMMEDIATE 模式下，約束是在每條語句后面進行檢查的。 在 DEFERRED 模式下，一直到事務提交時才檢查約束。

 在你把約束的模式修改成 IMMEDIATE 之后，新的約束模式是反作用式地生效的： 任何尚在等待的，需要在事務結束檢查地數據修改的約束（在使用 DEFERRED 的時候）都將在執行 SET CONSTRAINTS  命令的時候馬上檢查。

 從創建的時候開始，一個約束總是表現為下面三個特性之一： INITIALLY DEFERRED, INITIALLY IMMEDIATE DEFERRABLE, 或 INITIALLY IMMEDIATE NOT DEFERRABLE。 第三個特性不會受 SET CONSTRAINTS影響。

 目前，只有外鍵約束被這個設置影響。檢查和***約束總是立即檢查的而不是可推遲的。  

NOTES 注意

 這個命令只在當前事務里修改約束的行為。因此，如果你在事務塊外面 （BEGIN/COMMIT 對）執行這個命令， 它將顯得沒有任何作用那樣。如果你希望不在每個事務中發出 SET CONSTRAINTS 也能修改約束的行為， 那么在創建這些約束的時候聲明 INITIALLY DEFERRED 或者 INITIALLY IMMEDIATE。  

COMPATIBILITY 兼容性

 這條命令與 SQL 標準里定義的行為兼容，只不過，在 PostgreSQL 里，它只適用于外鍵約束。  

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NAME

SET CONSTRAINTS - set the constraint mode of the current transaction

SYNOPSIS

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

DESCRIPTION

SET CONSTRAINTS sets the behavior of constraint evaluation in the current transaction. In IMMEDIATE mode, constraints are checked at the end of each statement. In DEFERRED mode, constraints are not checked until transaction commit.

When you change the mode of a constraint to be IMMEDIATE, the new constraint mode takes effect retroactively: any outstanding data modifications that would have been checked at the end of the transaction (when using DEFERRED) are instead checked during the execution of the SET CONSTRAINTS command.

Upon creation, a constraint is always give one of three characteristics: INITIALLY DEFERRED, INITIALLY IMMEDIATE DEFERRABLE, or INITIALLY IMMEDIATE NOT DEFERRABLE. The third class is not affected by the SET CONSTRAINTS command.

Currently, only foreign key constraints are affected by this setting. Check and unique constraints are always effectively initially immediate not deferrable.  

NOTES

This command only alters the behavior of constraints within the current transaction. Thus, if you execute this command outside of a transaction block (BEGIN/COMMIT pair), it will not appear to have any effect. If you wish to change the behavior of a constraint without needing to issue a SET CONSTRAINTS command in every transaction, specify INITIALLY DEFERRED or INITIALLY IMMEDIATE when you create the constraint.  

COMPATIBILITY

This command complies with the behavior defined in the SQL standard, except for the limitation that, in PostgreSQL, it only applies to foreign-key constraints.