database – 当我们需要从触发程序返回值时？

2019年7月28日 181次阅读

有关触发器过程的文档(
https://www.postgresql.org/docs/9.6/static/plpgsql-trigger.html)说：“触发器函数必须返回NULL或记录/行”.

例

CREATE TRIGGER my_trigger
AFTER INSERT ON table_name
FOR EACH ROW EXECUTE PROCEDURE some_trigger_function();

假设some_trigger_function()返回记录/行,我明白程序体是在这个事件上调用并执行的,在这个表上,依此类推……但是在哪里返回值这个程序？

问题是：当我们使用那个返回值时？为什么我们需要这个价值呢？你能给出一些使用这个返回值的例子/解释吗？

最佳答案 阅读
the documentation：

Trigger functions invoked by per-statement triggers should always
return NULL. Trigger functions invoked by per-row triggers can return
a table row (a value of type HeapTuple) to the calling executor, if
they choose. A row-level trigger fired before an operation has the
following choices:

  • It can return NULL to skip the operation for the current row. This instructs the executor to not perform the row-level operation that
    invoked the trigger (the insertion, modification, or deletion of a
    particular table row).

  • For row-level INSERT and UPDATE triggers only, the returned row becomes the row that will be inserted or will replace the row being
    updated. This allows the trigger function to modify the row being
    inserted or updated.

A row-level BEFORE trigger that does not intend to cause either of
these behaviors must be careful to return as its result the same row
that was passed in (that is, the NEW row for INSERT and UPDATE
triggers, the OLD row for DELETE triggers).

A row-level INSTEAD OF trigger should either return NULL to indicate
that it did not modify any data from the view’s underlying base
tables, or it should return the view row that was passed in (the NEW
row for INSERT and UPDATE operations, or the OLD row for DELETE
operations). A nonnull return value is used to signal that the trigger
performed the necessary data modifications in the view. This will
cause the count of the number of rows affected by the command to be
incremented. For INSERT and UPDATE operations, the trigger may modify
the NEW row before returning it. This will change the data returned by
INSERT RETURNING or UPDATE RETURNING, and is useful when the view will
not show exactly the same data that was provided.

The return value is ignored for row-level triggers fired after an
operation, and so they can return NULL.

以下示例显示如何有条件地中止触发器中的插入：

create table my_table(id int);

-- do not insert rows with id > 10
create or replace function before_insert_on_my_table()
returns trigger language plpgsql as $$
begin
    return case
        when new.id > 10 then null
        else new
    end;
end $$;

create trigger before_insert_on_my_table
before insert on my_table
for each row execute procedure before_insert_on_my_table();

insert into my_table
values (15), (10), (5), (20)
returning id;

 id 
----
 10
  5
(2 rows)
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