Postgresql 中文操作指南

CREATE TRIGGER

CREATE TRIGGER - 定义一个新触发器

CREATE TRIGGER — define a new trigger

Synopsis

CREATE [ OR REPLACE ] [ CONSTRAINT ] TRIGGER name { BEFORE | AFTER | INSTEAD OF } { event [ OR ... ] }
    ON table_name
    [ FROM referenced_table_name ]
    [ NOT DEFERRABLE | [ DEFERRABLE ] [ INITIALLY IMMEDIATE | INITIALLY DEFERRED ] ]
    [ REFERENCING { { OLD | NEW } TABLE [ AS ] transition_relation_name } [ ... ] ]
    [ FOR [ EACH ] { ROW | STATEMENT } ]
    [ WHEN ( condition ) ]
    EXECUTE { FUNCTION | PROCEDURE } function_name ( arguments )

where event can be one of:

    INSERT
    UPDATE [ OF column_name [, ... ] ]
    DELETE
    TRUNCATE

Description

CREATE TRIGGER 将创建一个新触发器。 CREATE OR REPLACE TRIGGER 将创建一个新触发器或替换一个现有的触发器。触发器将与指定的表、视图或外部表相关联,并在对该表执行某些操作时执行指定的函数 function_name

CREATE TRIGGER creates a new trigger. CREATE OR REPLACE TRIGGER will either create a new trigger, or replace an existing trigger. The trigger will be associated with the specified table, view, or foreign table and will execute the specified function function_name when certain operations are performed on that table.

要替换现有触发器的当前定义,请使用 CREATE OR REPLACE TRIGGER ,指定现有触发器的名称和父表。所有其他属性都将被替换。

To replace the current definition of an existing trigger, use CREATE OR REPLACE TRIGGER, specifying the existing trigger’s name and parent table. All other properties are replaced.

触发器可以指定在对行执行操作之前(检查约束和尝试执行 INSERTUPDATEDELETE 之前)、在操作完成后(检查约束和完成 INSERTUPDATEDELETE 之后)、或在执行操作期间(针对视图上的插入、更新或删除)。如果触发器在事件之前或事件期间执行,触发器可以跳过当前行的操作,或更改正在插入的行(仅对 INSERTUPDATE 操作)。如果触发器在事件之后执行,则所有更改(包括其他触发器的作用)都对触发器“可见”。

The trigger can be specified to fire before the operation is attempted on a row (before constraints are checked and the INSERT, UPDATE, or DELETE is attempted); or after the operation has completed (after constraints are checked and the INSERT, UPDATE, or DELETE has completed); or instead of the operation (in the case of inserts, updates or deletes on a view). If the trigger fires before or instead of the event, the trigger can skip the operation for the current row, or change the row being inserted (for INSERT and UPDATE operations only). If the trigger fires after the event, all changes, including the effects of other triggers, are “visible” to the trigger.

标记为 FOR EACH ROW 的触发器针对操作修改的每一行调用一次。例如,影响到 10 行的 DELETE 将导致目标关系上的任何 ON DELETE 触发器被调用 10 次,针对每一行删除都调用一次。相比之下,标记为 FOR EACH STATEMENT 的触发器仅针对任何给定的操作执行一次,无论其修改多少行(尤其是,修改 0 行的操作仍会造成任何适用的 FOR EACH STATEMENT 触发器被执行)。

A trigger that is marked FOR EACH ROW is called once for every row that the operation modifies. For example, a DELETE that affects 10 rows will cause any ON DELETE triggers on the target relation to be called 10 separate times, once for each deleted row. In contrast, a trigger that is marked FOR EACH STATEMENT only executes once for any given operation, regardless of how many rows it modifies (in particular, an operation that modifies zero rows will still result in the execution of any applicable FOR EACH STATEMENT triggers).

指定为在 INSTEAD OF 触发器事件之前执行的触发器必须标记为 FOR EACH ROW ,并且只能在视图上定义。视图上的 BEFOREAFTER 触发器必须标记为 FOR EACH STATEMENT

Triggers that are specified to fire INSTEAD OF the trigger event must be marked FOR EACH ROW, and can only be defined on views. BEFORE and AFTER triggers on a view must be marked as FOR EACH STATEMENT.

此外,触发器可以被定义为在 TRUNCATE 之前执行,但只能是 FOR EACH STATEMENT

In addition, triggers may be defined to fire for TRUNCATE, though only FOR EACH STATEMENT.

下表总结了哪种类型的触发器可以使用在表、视图和外部表上:

The following table summarizes which types of triggers may be used on tables, views, and foreign tables:

此外,触发器定义可以指定一个布尔 WHEN 条件,该条件将被测试以查看是否应该触发触发器。在行级触发器中, WHEN 条件可以检查行的列的旧值和/或新值。语句级触发器也可以有 WHEN 条件,尽管此功能对于它们并不是很有用,因为该条件不能引用表中的任何值。

Also, a trigger definition can specify a Boolean WHEN condition, which will be tested to see whether the trigger should be fired. In row-level triggers the WHEN condition can examine the old and/or new values of columns of the row. Statement-level triggers can also have WHEN conditions, although the feature is not so useful for them since the condition cannot refer to any values in the table.

如果为同一事件定义了同种类型的多个触发器,它们将按照名称的字母顺序执行。

If multiple triggers of the same kind are defined for the same event, they will be fired in alphabetical order by name.

如果指定 CONSTRAINT 选项,此命令将创建一个 constraint trigger 。这与常规触发器相同,除非可以使用 SET CONSTRAINTS 调整触发器触发的时间。约束触发器必须是对普通表(非外部表)的 AFTER ROW 触发器。它们可以触发语句结尾引发触发事件或包含事务结尾;在后一种情况下称之为 deferred 。也可通过 SET CONSTRAINTS 使用 constraint trigger 立即执行挂起的延迟触发。当所实现的约束被违反时,约束触发器有望引发异常。

When the CONSTRAINT option is specified, this command creates a constraint trigger. This is the same as a regular trigger except that the timing of the trigger firing can be adjusted using SET CONSTRAINTS. Constraint triggers must be AFTER ROW triggers on plain tables (not foreign tables). They can be fired either at the end of the statement causing the triggering event, or at the end of the containing transaction; in the latter case they are said to be deferred. A pending deferred-trigger firing can also be forced to happen immediately by using SET CONSTRAINTS. Constraint triggers are expected to raise an exception when the constraints they implement are violated.

REFERENCING 选项允许收集 transition relations ,这些 transition relations 是包含当前 SQL 语句插入、删除或修改的所有行的行集。此功能让触发器可以查看该语句的全局视图,而不仅仅是一次查看一行。此选项仅允许用于不是约束触发器的 AFTER 触发器;此外,如果触发器是 UPDATE 触发器,它不能指定 column_name 列表。 OLD TABLE 只能指定一次,且只能用于可以在 UPDATEDELETE 上触发的触发器;它将创建一个过渡关系,其中包含语句更新或删除的所有行的 before-images 。类似地, NEW TABLE 只能指定一次,并且只能用于可以在 UPDATEINSERT 上触发的触发器;它将创建一个过渡关系,其中包含语句更新或插入的所有行的 after-images

The REFERENCING option enables collection of transition relations, which are row sets that include all of the rows inserted, deleted, or modified by the current SQL statement. This feature lets the trigger see a global view of what the statement did, not just one row at a time. This option is only allowed for an AFTER trigger that is not a constraint trigger; also, if the trigger is an UPDATE trigger, it must not specify a column_name list. OLD TABLE may only be specified once, and only for a trigger that can fire on UPDATE or DELETE; it creates a transition relation containing the before-images of all rows updated or deleted by the statement. Similarly, NEW TABLE may only be specified once, and only for a trigger that can fire on UPDATE or INSERT; it creates a transition relation containing the after-images of all rows updated or inserted by the statement.

SELECT 不修改任何行,因此你无法创建 SELECT 触发器。规则和视图可能会为看似需要 SELECT 触发器的问题提供可行的解决方案。

SELECT does not modify any rows so you cannot create SELECT triggers. Rules and views may provide workable solutions to problems that seem to need SELECT triggers.

请参阅 Chapter 39 以了解有关触发器的更多信息。

Refer to Chapter 39 for more information about triggers.

Parameters

  • name

    • The name to give the new trigger. This must be distinct from the name of any other trigger for the same table. The name cannot be schema-qualified — the trigger inherits the schema of its table. For a constraint trigger, this is also the name to use when modifying the trigger’s behavior using SET CONSTRAINTS.

  • BEFORE_AFTER_INSTEAD OF

    • Determines whether the function is called before, after, or instead of the event. A constraint trigger can only be specified as AFTER.

  • event

    • One of INSERT, UPDATE, DELETE, or TRUNCATE; this specifies the event that will fire the trigger. Multiple events can be specified using OR, except when transition relations are requested.

    • For UPDATE events, it is possible to specify a list of columns using this syntax:

UPDATE OF column_name1 [, column_name2 ... ]
  • The trigger will only fire if at least one of the listed columns is mentioned as a target of the UPDATE command or if one of the listed columns is a generated column that depends on a column that is the target of the UPDATE.

  • INSTEAD OF UPDATE events do not allow a list of columns. A column list cannot be specified when requesting transition relations, either.

    • table_name

  • The name (optionally schema-qualified) of the table, view, or foreign table the trigger is for.

    • referenced_table_name

  • The (possibly schema-qualified) name of another table referenced by the constraint. This option is used for foreign-key constraints and is not recommended for general use. This can only be specified for constraint triggers.

    • DEFERRABLE_NOT DEFERRABLE_INITIALLY IMMEDIATE__INITIALLY DEFERRED

  • The default timing of the trigger. See the CREATE TABLE documentation for details of these constraint options. This can only be specified for constraint triggers.

    • REFERENCING

  • This keyword immediately precedes the declaration of one or two relation names that provide access to the transition relations of the triggering statement.

    • OLD TABLE__NEW TABLE

  • This clause indicates whether the following relation name is for the before-image transition relation or the after-image transition relation.

    • transition_relation_name

  • The (unqualified) name to be used within the trigger for this transition relation.

    • FOR EACH ROW__FOR EACH STATEMENT

  • This specifies whether the trigger function should be fired once for every row affected by the trigger event, or just once per SQL statement. If neither is specified, FOR EACH STATEMENT is the default. Constraint triggers can only be specified FOR EACH ROW.

    • condition

  • A Boolean expression that determines whether the trigger function will actually be executed. If WHEN is specified, the function will only be called if the condition returns true. In FOR EACH ROW triggers, the WHEN condition can refer to columns of the old and/or new row values by writing OLD._column_name or _NEW._column_name respectively. Of course, _INSERT triggers cannot refer to OLD and DELETE triggers cannot refer to NEW.

  • INSTEAD OF triggers do not support WHEN conditions.

  • Currently, WHEN expressions cannot contain subqueries.

  • Note that for constraint triggers, evaluation of the WHEN condition is not deferred, but occurs immediately after the row update operation is performed. If the condition does not evaluate to true then the trigger is not queued for deferred execution.

    • function_name

  • A user-supplied function that is declared as taking no arguments and returning type trigger, which is executed when the trigger fires.

  • In the syntax of CREATE TRIGGER, the keywords FUNCTION and PROCEDURE are equivalent, but the referenced function must in any case be a function, not a procedure. The use of the keyword PROCEDURE here is historical and deprecated.

    • arguments

  • An optional comma-separated list of arguments to be provided to the function when the trigger is executed. The arguments are literal string constants. Simple names and numeric constants can be written here, too, but they will all be converted to strings. Please check the description of the implementation language of the trigger function to find out how these arguments can be accessed within the function; it might be different from normal function arguments.

Notes

要创建或替换表上的触发器,用户必须具有 TRIGGER 表权限。用户还必须具有 EXECUTE 触发器函数权限。

To create or replace a trigger on a table, the user must have the TRIGGER privilege on the table. The user must also have EXECUTE privilege on the trigger function.

使用 DROP TRIGGER 来删除触发器。

Use DROP TRIGGER to remove a trigger.

在分区表上创建行级触发器会导致在每个现有分区中创建相同的“克隆”触发器;稍后创建或附加的任何分区也将具有相同的触发器。如果子分区上已经存在名称冲突的触发器,则会发生错误,但 CREATE OR REPLACE TRIGGER 除外,在这种情况下该触发器将被克隆触发器替换。当分区从其父分区分离时,其克隆触发器将被删除。

Creating a row-level trigger on a partitioned table will cause an identical “clone” trigger to be created on each of its existing partitions; and any partitions created or attached later will have an identical trigger, too. If there is a conflictingly-named trigger on a child partition already, an error occurs unless CREATE OR REPLACE TRIGGER is used, in which case that trigger is replaced with a clone trigger. When a partition is detached from its parent, its clone triggers are removed.

列指定触发器(使用 UPDATE OF _column_name_ 语法定义的一个触发器)将在其任何列被列为 UPDATE 命令的 SET 列表中的目标时触发。即使未触发触发器,列的值也可能发生改变,因为未考虑对行的内容由 BEFORE UPDATE 触发器所做的更改。相反,即使列的值未改变,诸如 UPDATE …​ SET x = x …​ 的命令也会触发列 x 上的触发器。

A column-specific trigger (one defined using the UPDATE OF _column_name_ syntax) will fire when any of its columns are listed as targets in the UPDATE command’s SET list. It is possible for a column’s value to change even when the trigger is not fired, because changes made to the row’s contents by BEFORE UPDATE triggers are not considered. Conversely, a command such as UPDATE …​ SET x = x …​ will fire a trigger on column x, even though the column’s value did not change.

BEFORE 触发器中, WHEN 条件在函数被或将要执行前立即进行评估,因此使用 WHEN 与在触发器函数的开头测试同一条件从本质上来说并无不同。请特别注意条件看到的 NEW 行是当前值,可能已被早期触发器修改过。另外,不允许 BEFORE 触发器的 WHEN 条件检查 NEW 行的系统列(例如 ctid ),因为这些列尚未设置。

In a BEFORE trigger, the WHEN condition is evaluated just before the function is or would be executed, so using WHEN is not materially different from testing the same condition at the beginning of the trigger function. Note in particular that the NEW row seen by the condition is the current value, as possibly modified by earlier triggers. Also, a BEFORE trigger’s WHEN condition is not allowed to examine the system columns of the NEW row (such as ctid), because those won’t have been set yet.

AFTER 触发器中, WHEN 条件在行更新发生之后立即进行评估,并确定是否将事件排队在语句末尾触发触发器。因此,当 AFTER 触发器的 WHEN 条件未返回真值时,无需排队事件或在语句末尾重新提取行。如果只需要对少数行触发器后,此操作可以显著加快修改了多行的语句的速度。

In an AFTER trigger, the WHEN condition is evaluated just after the row update occurs, and it determines whether an event is queued to fire the trigger at the end of statement. So when an AFTER trigger’s WHEN condition does not return true, it is not necessary to queue an event nor to re-fetch the row at end of statement. This can result in significant speedups in statements that modify many rows, if the trigger only needs to be fired for a few of the rows.

在某些情况下,单个 SQL 命令有可能触发多种类型的触发器。例如,带有 ON CONFLICT DO UPDATE 子句的 INSERT 可能导致插入和更新操作,因此它将根据需要触发两种类型的触发器。提供给触发器的转换关系特定于其事件类型;因此, INSERT 触发器会仅看到插入的行,而 UPDATE 触发器会仅看到更新的行。

In some cases it is possible for a single SQL command to fire more than one kind of trigger. For instance an INSERT with an ON CONFLICT DO UPDATE clause may cause both insert and update operations, so it will fire both kinds of triggers as needed. The transition relations supplied to triggers are specific to their event type; thus an INSERT trigger will see only the inserted rows, while an UPDATE trigger will see only the updated rows.

由外键强制操作导致的行更新或删除(例如 ON UPDATE CASCADEON DELETE SET NULL )被视为导致它们 SQL 命令的一部分(请注意,此类操作绝不会被递延)。受影响表上相关的触发器将会被触发,因此这是 SQL 命令有可能触发与自己的类型不直接匹配的触发器的另一种方式。在简单情况下,请求转换关系的触发器会看到单个原始 SQL 命令导致的表中的所有更改作为单个转换关系。但是,在某些情况下,请求转换关系的 AFTER ROW 触发器会导致由单个 SQL 命令触发的外键强制操作被分割为多个步骤,每个步骤都有自己的转换关系。在这种情况下,任何语句级触发器都会在每个转换关系集创建时触发一次,从而确保触发器在转换关系中对每行受影响的行都看到一次且仅看到一次。

Row updates or deletions caused by foreign-key enforcement actions, such as ON UPDATE CASCADE or ON DELETE SET NULL, are treated as part of the SQL command that caused them (note that such actions are never deferred). Relevant triggers on the affected table will be fired, so that this provides another way in which an SQL command might fire triggers not directly matching its type. In simple cases, triggers that request transition relations will see all changes caused in their table by a single original SQL command as a single transition relation. However, there are cases in which the presence of an AFTER ROW trigger that requests transition relations will cause the foreign-key enforcement actions triggered by a single SQL command to be split into multiple steps, each with its own transition relation(s). In such cases, any statement-level triggers that are present will be fired once per creation of a transition relation set, ensuring that the triggers see each affected row in a transition relation once and only once.

对视图的语句级触发器仅在视图上的操作由行级 INSTEAD OF 触发器处理时才触发。如果操作由 INSTEAD 规则处理,则规则发出的任何语句都会在命名视图的原始语句中执行,因此将会触发的触发器是替换语句中命名的表上的触发器。类似地,如果视图是自动可更新的,则该操作将通过自动将语句重写为对视图基表的动作来处理,因此基表的语句级触发器是触发的。

Statement-level triggers on a view are fired only if the action on the view is handled by a row-level INSTEAD OF trigger. If the action is handled by an INSTEAD rule, then whatever statements are emitted by the rule are executed in place of the original statement naming the view, so that the triggers that will be fired are those on tables named in the replacement statements. Similarly, if the view is automatically updatable, then the action is handled by automatically rewriting the statement into an action on the view’s base table, so that the base table’s statement-level triggers are the ones that are fired.

修改分区表或有继承子表的表会触发附加到显式命名的表的语句级触发器,但不会触发其分区或子表的语句级触发器。相比之下,行级触发器会触发在受影响的分区或子表中的行上,即使它们未在查询中显式命名。如果已经使用 REFERENCING 子句命名的转换关系定义了语句级触发器,那么受影响的所有分区或子表都可见行图像。对于继承子表,行图像仅包含触发器附加到的表中存在的列。

Modifying a partitioned table or a table with inheritance children fires statement-level triggers attached to the explicitly named table, but not statement-level triggers for its partitions or child tables. In contrast, row-level triggers are fired on the rows in affected partitions or child tables, even if they are not explicitly named in the query. If a statement-level trigger has been defined with transition relations named by a REFERENCING clause, then before and after images of rows are visible from all affected partitions or child tables. In the case of inheritance children, the row images include only columns that are present in the table that the trigger is attached to.

当前,无法对分区或继承子表定义带有转换关系的行级触发器。同样,分区表上的触发器可能不 INSTEAD OF

Currently, row-level triggers with transition relations cannot be defined on partitions or inheritance child tables. Also, triggers on partitioned tables may not be INSTEAD OF.

当前,不支持对约束触发器的 OR REPLACE 选项。

Currently, the OR REPLACE option is not supported for constraint triggers.

不建议在已经对触发器的表执行过更新操作的事务中替换现有的触发器。已做出的触发触发决策或触发决策的部分不会被再次考虑,因此效果可能会令人惊讶。

Replacing an existing trigger within a transaction that has already performed updating actions on the trigger’s table is not recommended. Trigger firing decisions, or portions of firing decisions, that have already been made will not be reconsidered, so the effects could be surprising.

有一些内置的触发器函数可以用来解决常见问题,无需编写触发器代码;请参阅 Section 9.28

There are a few built-in trigger functions that can be used to solve common problems without having to write your own trigger code; see Section 9.28.

Examples

每当对表 accounts 的行进行更新时执行函数 check_account_update

Execute the function check_account_update whenever a row of the table accounts is about to be updated:

CREATE TRIGGER check_update
    BEFORE UPDATE ON accounts
    FOR EACH ROW
    EXECUTE FUNCTION check_account_update();

修改该触发器定义,仅在 UPDATE 命令中指定列 balance 作为目标时才执行此函数:

Modify that trigger definition to only execute the function if column balance is specified as a target in the UPDATE command:

CREATE OR REPLACE TRIGGER check_update
    BEFORE UPDATE OF balance ON accounts
    FOR EACH ROW
    EXECUTE FUNCTION check_account_update();

这个表单仅在列 balance 实际更改值时执行此函数:

This form only executes the function if column balance has in fact changed value:

CREATE TRIGGER check_update
    BEFORE UPDATE ON accounts
    FOR EACH ROW
    WHEN (OLD.balance IS DISTINCT FROM NEW.balance)
    EXECUTE FUNCTION check_account_update();

调用一个函数来记录 accounts 的更新情况,但仅限在更改某些内容时:

Call a function to log updates of accounts, but only if something changed:

CREATE TRIGGER log_update
    AFTER UPDATE ON accounts
    FOR EACH ROW
    WHEN (OLD.* IS DISTINCT FROM NEW.*)
    EXECUTE FUNCTION log_account_update();

为每一行执行函数 view_insert_row 以向视图的基础表中插入行:

Execute the function view_insert_row for each row to insert rows into the tables underlying a view:

CREATE TRIGGER view_insert
    INSTEAD OF INSERT ON my_view
    FOR EACH ROW
    EXECUTE FUNCTION view_insert_row();

为每一行执行函数 check_transfer_balances_to_zero 以确认 transfer 行的偏移量为零:

Execute the function check_transfer_balances_to_zero for each statement to confirm that the transfer rows offset to a net of zero:

CREATE TRIGGER transfer_insert
    AFTER INSERT ON transfer
    REFERENCING NEW TABLE AS inserted
    FOR EACH STATEMENT
    EXECUTE FUNCTION check_transfer_balances_to_zero();

为每一行执行函数 check_matching_pairs 以确认同时 (由同一语句) 对匹配对进行更改:

Execute the function check_matching_pairs for each row to confirm that changes are made to matching pairs at the same time (by the same statement):

CREATE TRIGGER paired_items_update
    AFTER UPDATE ON paired_items
    REFERENCING NEW TABLE AS newtab OLD TABLE AS oldtab
    FOR EACH ROW
    EXECUTE FUNCTION check_matching_pairs();

Section 39.4 中包含一个用 C 编写的触发器函数的完整示例。

Section 39.4 contains a complete example of a trigger function written in C.

Compatibility

PostgreSQL 中的 CREATE TRIGGER 语句实现了 SQL 标准的一个子集。目前尚缺少以下功能:

The CREATE TRIGGER statement in PostgreSQL implements a subset of the SQL standard. The following functionalities are currently missing:

SQL 规定应该按照创建时间顺序触发多个触发器。PostgreSQL 使用名称顺序,这被认为更方便。

SQL specifies that multiple triggers should be fired in time-of-creation order. PostgreSQL uses name order, which was judged to be more convenient.

SQL 规定,在级联删除中对 BEFORE DELETE 触发器触发 after 级联 DELETE 完成。PostgreSQL 行为是 BEFORE DELETE 总是先于删除操作触发,即使是级联操作。这被认为更一致。如果 BEFORE 触发器在由引用操作引起的更新期间修改行或阻止更新,也会有非标准行为。这可能导致约束冲突或存储的数据不符合引用约束。

SQL specifies that BEFORE DELETE triggers on cascaded deletes fire after the cascaded DELETE completes. The PostgreSQL behavior is for BEFORE DELETE to always fire before the delete action, even a cascading one. This is considered more consistent. There is also nonstandard behavior if BEFORE triggers modify rows or prevent updates during an update that is caused by a referential action. This can lead to constraint violations or stored data that does not honor the referential constraint.

使用 OR 为单个触发器指定多个操作的能力是 SQL 标准中 PostgreSQL 扩展。

The ability to specify multiple actions for a single trigger using OR is a PostgreSQL extension of the SQL standard.

TRUNCATE 触发触发器的能力是 SQL 标准中 PostgreSQL 扩展,定义视图中语句级触发器的能力也是如此。

The ability to fire triggers for TRUNCATE is a PostgreSQL extension of the SQL standard, as is the ability to define statement-level triggers on views.

CREATE CONSTRAINT TRIGGER 是 SQL 标准中 PostgreSQL 扩展。 OR REPLACE 选项也是如此。

CREATE CONSTRAINT TRIGGER is a PostgreSQL extension of the SQL standard. So is the OR REPLACE option.