确定Postgresql中的触发源

Question

你认为可以确定PostgreSQL中触发器执行的来源吗？我们假设我有两个表如下:

CREATE TABLE tbl1 (id bigserial NOT NULL PRIMARY KEY,
    name text NOT NULL);
CREATE TABLE tbl2 (id bigserial NOT NULL PRIMARY KEY,
    owner bigint NOT NULL REFERENCES tbl1(id) ON DELETE CASCADE,
    prop text NOT NULL);

其中tbl2使用"ON DELETE CASCADE"引用tbl1.

此外,让我们在tbl2上定义一个触发器,它在删除ROW后执行:

CREATE FUNCTION test_fn() RETURNS trigger AS $$
BEGIN
    RAISE NOTICE 'test_fn()';
    RETURN NULL;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER test_delete AFTER DELETE ON tbl2
    FOR EACH ROW EXECUTE PROCEDURE test_fn();

在删除tbl2中的ROW之后始终执行触发器,如果​​直接删除行或通过级联删除行,则独立执行.例如,下面的两个语句最终触发了触发器:

DELETE FROM tbl1 WHERE id = 1;
DELETE FROM tbl2 WHERE id = 1;

在test_fn()里面,有可能区分这两种情况吗？即弄清楚为什么行被删除？我试图确定使用堆栈的原因(即使用GET DIAGNOSTICS stack = PG_CONTEXT),但没有任何结果.

有人可以帮我吗？非常感谢提前

Answer 1

操作的上下文（即级联删除还是简单删除）应该存储在某个地方。您可以使用自定义参数来实现此目的。tbl1当执行级联删除（从表）时，触发器按以下顺序触发：

trigger before delete on tbl1
trigger before delete on tbl2
trigger after delete on tbl1
trigger after delete on tbl2

因此，您需要 table 上的两个触发器（删除之前和删除之后）tbl1和 table 上的删除之前的触发器tbl2。

在 上创建两个触发器tbl1。on在触发前函数和off触发后函数中设置自定义参数：

create or replace function tbl1_trigger_before_delete()
returns trigger language plpgsql as $$
begin
    set tbl1.cascade to on;
    return old;
end $$;

create or replace function tbl1_trigger_after_delete()
returns trigger language plpgsql as $$
begin
    set tbl1.cascade to off;
    return null;
end $$;

create trigger tbl1_trigger_before_delete
before delete on tbl1
for each row execute procedure tbl1_trigger_before_delete();

create trigger tbl1_trigger_after_delete
after delete on tbl1
for each row execute procedure tbl1_trigger_after_delete();

在tbl2触发功能中检查参数的当前值。如果参数尚未设置，则需要异常块：

create or replace function tbl2_trigger_before_delete()
returns trigger language plpgsql as $$
begin
    begin
        if current_setting('tbl1.cascade') = 'on' then
            raise notice 'cascaded';
        else
            raise exception '';
        end if;
    exception when others then
        raise notice 'not cascaded';
    end;
    return old;
end $$;

create trigger tbl2_trigger_before_delete
before delete on tbl2
for each row execute procedure tbl2_trigger_before_delete();

测试：

insert into tbl1 values
(1, '1'),
(2, '2');

insert into tbl2 values
(1, 1, '1'),
(2, 1, '2'),
(3, 2, '3'),
(4, 2, '4');

delete from tbl1 where id = 1;
NOTICE:  cascaded
NOTICE:  cascaded
DELETE 1

delete from tbl2 where owner = 2;
NOTICE:  not cascaded
NOTICE:  not cascaded
DELETE 2    

替代解决方案。

当在级联删除上下文中执行表上的删除之前的触发器时tbl2，诊断值 PG_EXCEPTION_CONTEXT 将设置为某个字符串，并且当删除不是级联时它为空：

create or replace function tbl2_trigger_before_delete()
returns trigger language plpgsql as $$
declare
    context text;
begin
    begin
        raise exception '';
    exception when others then
        GET STACKED DIAGNOSTICS context := PG_EXCEPTION_CONTEXT;
    end;
    if context = '' then
        raise notice 'not cascaded';
    else
        raise notice 'cascaded';
    end if;
    return old;
end $$;

create trigger tbl2_trigger_before_delete
before delete on tbl2
for each row execute procedure tbl2_trigger_before_delete();

这个解决方案可能是有问题的，因为它仅来自测试，这种行为没有在任何地方记录。