当我们需要来自触发器程序的 return 值时？

Question

有关触发程序的文档 (https://www.postgresql.org/docs/9.6/static/plpgsql-trigger.html) 说："A trigger function must return either NULL or a record/row".

例子

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

假设 some_trigger_function() returns record/row，我知道程序主体在这个事件上被调用和执行，在这个 table 上，依此类推...但是returns 值这个程序在哪里？

问题是：我们什么时候使用那个返回值？为什么我们需要这个值？能不能给littleexample/explanation，它用的是这个返回值？

Answer 1

读入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.

以下示例显示了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)

当我们需要来自触发器程序的 return 值时？

When we need return value from trigger procedure?

database

postgresql

triggers

postgresql-9.6