根据外部表的最后一个条目插入记录

Question

我有以下简单的表关系：

CREATE TABLE PRODUCT (
  ID                BIGSERIAL,
  TYPE              VARCHAR(24),
  TITLE             VARCHAR(128),
  PRIMARY KEY (ID)
);

CREATE TABLE EVENT (
  ID               BIGSERIAL,
  STATE            VARCHAR(64) NOT NULL,
  DATETIME         TIMESTAMP   NOT NULL,
  PRODUCT_ID        BIGINT,
  FOREIGN KEY (PRODUCT_ID) REFERENCES PRODUCT(ID),
  PRIMARY KEY (ID)
);

Event中的“STATE”，由Java Enum转换而来，可以是：INITIALIZED、AVAILABLE、PROCESSED等

我想要执行以下操作：如果产品的最后一个事件条目具有状态“已处理”，则在状态为“可用”的 EVENT 表中创建一个新插入。这必须适用于所有产品。

如您所见，事件表引用了产品。我看过：Insert, on duplicate update in PostgreSQL?

但是想不通。

Answer 1

如果您的数据是按顺序插入的，我更倾向于使用id 进行比较，如下所示：

insert into event (state, datetime, product_id)
    select 'AVAILABLE', current_timestamp, product_id
    from event e
    where e.state = 'PROCESSED' and
          not exists (select e2.state
                      from event e2
                      where e2.product_id = e.product_id and
                            e2.id > e.id  -- you can use timestamp
                     );

我更感兴趣的是你说过state 是enum。 Postgres确实支持enumerated types，而你没有。但是，您想要做的通常是使用检查约束来处理：

CREATE TABLE EVENT (
  ID               BIGSERIAL,
  STATE            VARCHAR(64) NOT NULL,
  DATETIME         TIMESTAMP   NOT NULL,
  PRODUCT_ID        BIGINT,
  FOREIGN KEY (PRODUCT_ID) REFERENCES PRODUCT(ID),
  PRIMARY KEY (ID),
  CONSTRAINT CHECK_EVENT_STATE CHECK (STATE IN ('INITIALIZED', 'AVAILABLE', 'PROCESSED'))
);

Answer 2

INSERT INTO EVENT( STATE, DATETIME, PRODUCT_ID )
SELECT 'AVAILABLE', current_timestamp, product_id
FROM EVENT e
WHERE PRODUCT_ID = 123
  AND e.STATE = 'PROCESSED'
  AND NOT EXISTS (
     SELECT 'anything' FROM event e1
     WHERE e1.PRODUCT_ID = e.PRODUCT_ID
       AND e1.DATETIME > e.DATETIME
);

---

如果该命令要在多线程/多用户环境下运行，那么整个事务必须由这四个命令组成，否则可能会出现重复记录：

BEGIN TRANSACTION;

/* lock the parent record */
PERFORM id FROM PRODUCT WHERE id = 123 FOR UPDATE;

INSERT INTO EVENT( STATE, DATETIME, PRODUCT_ID )
    SELECT 'AVAILABLE', current_timestamp, product_id
    FROM EVENT e
    WHERE PRODUCT_ID = 123
      AND e.STATE = 'PROCESSED'
      AND NOT EXISTS (
         SELECT 'anything' FROM event e1
         WHERE e1.PRODUCT_ID = e.PRODUCT_ID
           AND e1.DATETIME > e.DATETIME
    );

COMMIT;