postgres中级联删除的顺序

Question

我遇到了一个问题，这让我怀疑 Postgres 仅在 在它已经删除了原始行之后才从依赖表中删除行（ON DELETE CASCADE）。

我有这些表：

CREATE TABLE IF NOT EXISTS function (
    id UUID PRIMARY KEY,
    level VARCHAR(4) NOT NULL CHECK (level IN ('ORG', 'DEP', 'GRP', 'SESS')),
    name VARCHAR(64) NOT NULL UNIQUE,
    type VARCHAR(15) NOT NULL CHECK (type IN ('SYSTEM', 'SYS-AUTO-ASSIGN', 'CUSTOM'))
);

CREATE TABLE IF NOT EXISTS function_inclusion (
    super_function UUID REFERENCES function (id) ON DELETE CASCADE,
    sub_function UUID REFERENCES function (id) ON DELETE CASCADE,
    UNIQUE (super_function, sub_function)
);

我在 function_inclusion 表上创建了一个触发器（在删除之前）：

CREATE OR REPLACE FUNCTION trg_function_inclusion_del_bef()
RETURNS trigger AS
$func$
DECLARE
    function_type VARCHAR(15);
BEGIN
    SELECT type INTO function_type FROM function WHERE id = OLD.super_function;
    RAISE NOTICE 'function_type: %', function_type;
    -- do stuff based on the function_type of the super_function
    CASE
        WHEN function_type = 'SYSTEM' OR function_type = 'SYS-AUTO-ASSIGN' THEN
            -- (do stuff)
        WHEN function_type = 'CUSTOM' THEN
            -- (do stuff)
        ELSE RAISE EXCEPTION 'The function % doesn''t have a correct type', OLD.super_function;
    END CASE;
    RETURN OLD;
END
$func$  
LANGUAGE plpgsql;

DROP TRIGGER IF EXISTS function_inclusion_delete_before ON function_inclusion CASCADE;

CREATE TRIGGER function_inclusion_delete_before
BEFORE DELETE ON function_inclusion
FOR EACH ROW 
EXECUTE PROCEDURE trg_function_inclusion_del_bef();

假设我有 2 个函数和一个 function_inclusion：

INSERT INTO function (id, level, name, type)
VALUES ('abcf3dbc-9433-4b73-b9c1-f00745dc1175', 'DEP', 'custom-function-1', 'CUSTOM');

INSERT INTO function (id, level, name, type)
VALUES ('360bde13-7953-49ed-a923-793b2d828d7e', 'DEP', 'custom-function-2', 'CUSTOM');

INSERT INTO function_inclusion (super_function, sub_function)
VALUES ('abcf3dbc-9433-4b73-b9c1-f00745dc1175', '360bde13-7953-49ed-a923-793b2d828d7e');

当我删除 super_function 时：

DELETE FROM function WHERE id = 'abcf3dbc-9433-4b73-b9c1-f00745dc1175';

我收到此错误：

NOTICE:  function_type: <NULL>
Query 1 ERROR: ERROR:  The function abcf3dbc-9433-4b73-b9c1-f00745dc1175 doesn't have a correct type
CONTEXT:  PL/pgSQL function trg_function_inclusion_del_bef() line 13 at RAISE
SQL statement "DELETE FROM ONLY "public"."function_inclusion" WHERE $1 OPERATOR(pg_catalog.=) "super_function""

因此，该函数似乎已被删除，我无法再通过 function_inclusion 上的触发器访问它。

我试图找到有关“ON DELETE CASCADE”的更多信息，但我读到的所有地方都只说“引用行被自动删除”，没有提及首先删除的是引用行还是被引用行。

postgres 在删除相关（引用）表中的行之前是否首先删除原始（引用）行？ 如果是这样，我怎样才能实现相同的东西，而不必在我的 function_inclusion 表中存储冗余数据？

Answer 1

以防万一其他人遇到同样的问题，我将发布我找到的解决方案。

问题是 - 正如我所怀疑的 - postgres 首先删除请求的行本身，然后从相关表（设置了“ON DELETE CASCADE”）中删除行。

我找到的解决方案有点复杂，在“功能”表上实现软删除（参见Cascading Soft Delete）

1. 首先我在“函数”表中添加了一个额外的字段deleted_at：

CREATE TABLE IF NOT EXISTS function (
    id UUID PRIMARY KEY,
    level VARCHAR(4) NOT NULL CHECK (level IN ('ORG', 'DEP', 'GRP', 'SESS')),
    name VARCHAR(64) NOT NULL UNIQUE,
    type VARCHAR(15) NOT NULL CHECK (type IN ('SYSTEM', 'SYS-AUTO-ASSIGN', 'CUSTOM')),
    deleted_at TIMESTAMPTZ DEFAULT NULL
);

此表包含所有“功能”行，包括已（软）删除的行。 从现在开始，我们将需要使用“SELECT FROM ONLY 函数”来选择未删除的行。

1. 然后我创建了一个继承表“function_deleted”：

CREATE TABLE IF NOT EXISTS function_deleted () INHERITS(function);

插入此表的行也将在表“函数”中找到。 要查找已删除的行，我们需要使用“SELECT FROM function_deleted”。

1. 然后我创建了一个通用触发器函数，用于从任何表中软删除一行：

CREATE OR REPLACE FUNCTION tr_soft_delete_row()
RETURNS TRIGGER AS 
$$
BEGIN
    IF (TG_OP = 'UPDATE' AND NEW.deleted_at IS NOT NULL) THEN
        EXECUTE format('DELETE FROM %I.%I WHERE id = $1', TG_TABLE_SCHEMA, TG_TABLE_NAME) USING OLD.id;
        RETURN OLD;
    END IF;
    IF (TG_OP = 'DELETE') THEN
        IF (OLD.deleted_at IS NULL) THEN
            OLD.deleted_at := timenow();
        END IF;
        EXECUTE format('INSERT INTO %I.%I SELECT $1.*', TG_TABLE_SCHEMA, TG_TABLE_NAME || '_deleted') USING OLD;
    END IF;
    RETURN OLD;
END;
$$ 
LANGUAGE plpgsql;

当在删除行时从触发器调用时，该函数会设置 deleted_at 字段并将该行插入“(table)_deleted”表中。

当在 delete_at 字段更新时从触发器调用时，此函数会删除该行（将自动变为软删除）

4. 然后我创建了一个触发器来调用这个软删除函数：

CREATE TRIGGER _soft_delete_function
AFTER
    UPDATE OF deleted_at 
    OR DELETE
    ON function
FOR EACH ROW
EXECUTE PROCEDURE tr_soft_delete_row();

（触发器名称前的下划线确保触发器将在任何其他触发器之前被调用）

5. 现在我像以前一样创建“function_inclusion”表：

CREATE TABLE IF NOT EXISTS function_inclusion (
    super_function UUID REFERENCES function (id) ON DELETE CASCADE,
    sub_function UUID REFERENCES function (id) ON DELETE CASCADE,
    UNIQUE (super_function, sub_function)
);

6. 以及该表的触发函数和触发器：

CREATE OR REPLACE FUNCTION trg_function_inclusion_del_aft()
RETURNS trigger AS
$func$
DECLARE
    function_type VARCHAR(15);
BEGIN
    SELECT type INTO function_type FROM ONLY function WHERE id = OLD.super_function;
    -- if the function doesn't exist then it's because it was just deleted
    --> find it in table "function_deleted"
    IF NOT FOUND THEN
        SELECT type INTO function_type FROM function_deleted WHERE id = OLD.super_function 
        ORDER BY deleted_at DESC NULLS LAST LIMIT 1;
    END IF;
    RAISE NOTICE 'function_type: %', function_type;
    -- do stuff based on the function_type of the super_function
    CASE
        WHEN function_type = 'SYSTEM' OR function_type = 'SYS-AUTO-ASSIGN' THEN
            -- (do stuff)
        WHEN function_type = 'CUSTOM' THEN
            -- (do stuff)
        ELSE RAISE EXCEPTION 'The function % doesn''t have a correct type', OLD.super_function;
    END CASE;
    RETURN OLD;
END
$func$  
LANGUAGE plpgsql;

CREATE TRIGGER function_inclusion_delete_after
AFTER DELETE ON function_inclusion
FOR EACH ROW 
EXECUTE PROCEDURE trg_function_inclusion_del_aft();

1. 现在，当我创建函数和函数包含时：

INSERT INTO function (id, level, name, type)
VALUES ('abcf3dbc-9433-4b73-b9c1-f00745dc1175', 'DEP', 'custom-function-1', 'CUSTOM');

INSERT INTO function (id, level, name, type)
VALUES ('360bde13-7953-49ed-a923-793b2d828d7e', 'DEP', 'custom-function-2', 'CUSTOM');

INSERT INTO function_inclusion (super_function, sub_function)
VALUES ('abcf3dbc-9433-4b73-b9c1-f00745dc1175', '360bde13-7953-49ed-a923-793b2d828d7e');

然后删除其中一个函数：

DELETE FROM ONLY function WHERE id = 'abcf3dbc-9433-4b73-b9c1-f00745dc1175';

我现在没有收到错误，并且我在引发的通知中获得了正确的信息（来自 function_inclusion 上的 AFTER DELETE 触发器）：

NOTICE:  function_type: CUSTOM
Query 1 OK: DELETE 1, 1 row affected

---

这种方法的一个缺点是，现在在我的 SQL 代码中，我需要记住使用“SELECT * FROM ONLY 函数”而不是“SELECT * FROM 函数”。