sql - 如何处理相互依赖的插入

Question

我有一个定义相互递归表的模型:

Answer
  questionId QuestionId
  text

Question
  text
  correct AnswerId

我需要做什么才能真正插入问题？我需要先知道正确答案是什么。但要插入答案，我需要知道它回答的是什么问题。

如果重要的话，我正在运行 Postgres。

DDL 是:

CREATE TABLE answer (
  id integer NOT NULL,                 -- answer id
  text character varying NOT NULL,     -- answer text
  question_id bigint NOT NULL          -- question id
);

CREATE TABLE question (
  id integer NOT NULL,                 -- question id
  question character varying NOT NULL, -- question text
  correct bigint NOT NULL,             -- correct answer
  solution character varying NOT NULL  -- solution text
);

ALTER TABLE ONLY answer ALTER COLUMN id SET DEFAULT nextval('answer_id_seq'::regclass);

ALTER TABLE ONLY answer
  ADD CONSTRAINT answer_question_id_fkey FOREIGN KEY (question_id) REFERENCES question(id);

ALTER TABLE ONLY question ALTER COLUMN id SET DEFAULT nextval('question_id_seq'::regclass);
ALTER TABLE ONLY question
  ADD CONSTRAINT question_correct_fkey FOREIGN KEY (correct) REFERENCES answer(id);
```sql

Answer 1

如果您在带有 data-modifying CTE 的单个语句中输入问题和答案，您甚至不需要 DEFERRABLE FK 约束。更不用说实际制作(或 SETting)它们 DEFERRED - 这会贵得多。

数据模型

首先我清理了你的数据模型:

CREATE TABLE question (
   question_id       serial PRIMARY KEY
 , correct_answer_id int  NOT NULL
 , question          text NOT NULL
 , solution          text NOT NULL
);

CREATE TABLE answer (
   answer_id   serial PRIMARY KEY
 , question_id int  NOT NULL REFERENCES question
 , answer      text NOT NULL
);

ALTER TABLE question ADD CONSTRAINT question_correct_answer_id_fkey
FOREIGN KEY (correct_answer_id) REFERENCES answer(answer_id);

• 不要使用非描述性的“id”或“text”(也是基本类型名称)作为列名。
• 将整数列放在最前面以提高空间效率。看:
  • Calculating and saving space in PostgreSQL
• bigint 不需要，integer 应该足够了。
• 使用 serial columns 简化您的模式定义.
• 定义主键。 PK 列自动为 NOT NULL。

解决方案

将主键生成委托(delegate)给序列(serial 列)后，我们可以使用 INSERT 的 RETURNING 子句获取自动生成的 ID陈述。但在这种特殊情况下，我们需要为每个 INSERT 提供 两个 ID，因此我使用 nextval() 获取其中一个 ID 以启动它。

WITH q AS (
   INSERT INTO question
          (correct_answer_id              , question, solution)
   VALUES (nextval('answer_answer_id_seq'), 'How?'  , 'DEFERRABLE FK & CTE')
   RETURNING correct_answer_id, question_id
   )
INSERT INTO answer
      (answer_id        , question_id, answer)
SELECT correct_answer_id, question_id, 'Use DEFERRABLE FK & CTE'
FROM   q;

我知道序列的名称 ('answer_answer_id_seq')，因为我查过它。这是默认名称。如果您不知道，请使用安全形式 @IMSoP provided in a comment :

nextval(pg_get_serial_sequence('answer', 'answer_id'))

DEFERRABLE 或 DEFERRED 约束？

The manual on SET CONSTRAINTS:

IMMEDIATE constraints are checked at the end of each statement.

我的解决方案是单个 语句。这就是为什么它在两个单独的语句会失败的情况下工作——是否包含在一个事务中。你需要 SET CONSTRAINTS ... DEFERRED; 就像 IMSoP first commented和 @Jaaz implemented in his answer .
但是，请注意下面几段的免责声明:

Uniqueness and exclusion constraints that have not been declared DEFERRABLE are also checked immediately.

所以 UNIQUE 和 EXCLUDE 需要是 DEFERRABLE 才能让 CTE 为它们工作。这包括 PRIMARY KEY 约束。 The documentation on CREATE TABLE has more details :

Non-deferred Uniqueness Constraints

When a UNIQUE or PRIMARY KEY constraint is not deferrable, PostgreSQL checks for uniqueness immediately whenever a row is inserted or modified. The SQL standard says that uniqueness should be enforced only at the end of the statement; this makes a difference when, for example, a single command updates multiple key values. To obtain standard-compliant behavior, declare the constraint as DEFERRABLE but not deferred (i.e., INITIALLY IMMEDIATE). Be aware that this can be significantly slower than immediate uniqueness checking.

我们在这个相关问题下对此进行了非常详细的讨论:

• Constraint defined DEFERRABLE INITIALLY IMMEDIATE is still DEFERRED?