如何确定 upsert 是否是 PostgreSQL 9.5+ UPSERT 的更新？

Question

如 Insert, on duplicate update in PostgreSQL?

中所述，可写 CTE 被认为是 9.5 之前的 UPSERT 的解决方案

可以使用以下可写 CTE 惯用语来执行 UPSERT，其中包含它是作为 UPDATE 还是 INSERT 结束的信息：

WITH
    update_cte AS (
        UPDATE t SET v =  WHERE id =  RETURNING 'updated'::text status
    ),
    insert_cte AS (
        INSERT INTO t(id, v) SELECT ,  WHERE NOT EXISTS
            (SELECT 1 FROM update_cte) RETURNING 'inserted'::text status
    )
 (SELECT status FROM update_cte) UNION (SELECT status FROM insert_cte)

此查询将 return "updated" 或 "inserted"，或者可能（很少）因违反约束而失败，如 https://dba.stackexchange.com/questions/78510/why-is-cte-open-to-lost-updates

中所述

是否可以使用 PostgreSQL 9.5+ 新的 "UPSERT" 语法实现类似的东西，从其优化中获益并避免可能的约束冲突？

Answer 1

在SQL Server MERGE语句中有$action那个returns字符串'INSERT', 'UPDATE', or 'DELETE'.

对于 Postgresql 我找不到 function/variable 为 RETURNING 做类似的事情。

一种解决方法是将列 is_updated 添加到您的 table:

DROP TABLE IF EXISTS tab;

CREATE TABLE tab(id INT PRIMARY KEY, col VARCHAR(100),
                 is_updated BOOLEAN DEFAULT false);
INSERT INTO tab(id, col) VALUES (1,'a'), (2, 'b');


-- main query
INSERT INTO tab(id, col)
VALUES (3, 'c'), (4, 'd'), (1,'aaaa')
ON CONFLICT (id) DO UPDATE SET col = EXCLUDED.col, is_updated = true
RETURNING id,col,
          CASE WHEN is_updated THEN 'UPDATED' ELSE 'INSERTED' END AS action;

Rextester Demo

输出：

╔════╦══════╦══════════╗
║ id ║ col  ║  action  ║
╠════╬══════╬══════════╣
║  3 ║ c    ║ INSERTED ║
║  4 ║ d    ║ INSERTED ║
║  1 ║ aaaa ║ UPDATED  ║
╚════╩══════╩══════════╝

Answer 2

从 WHERE 子句中的 , the result can be achieved by abusing settings and customized options with related functions 获取所需的 side-effect。

CREATE TABLE t(id INT PRIMARY KEY, v TEXT);

INSERT INTO t (id, v)
    SELECT , 
    WHERE 'inserted' = set_config('upsert.action', 'inserted', true)
    ON CONFLICT (id) DO UPDATE
        SET v = EXCLUDED.v
        WHERE 'updated' = set_config('upsert.action', 'updated', true)
RETURNING current_setting('upsert.action') AS "upsert.action";

set_config的第三个参数是is_local：true表示交易结束后设置消失。更准确地说，current_setting('upsert.action') 将 return NULL（并且不会抛出错误）直到会话结束。

Answer 3

我相信 xmax::text::int > 0 是最简单的技巧：

so=# DROP TABLE IF EXISTS tab;
NOTICE:  table "tab" does not exist, skipping
DROP TABLE
so=# CREATE TABLE tab(id INT PRIMARY KEY, col text);
CREATE TABLE
so=# INSERT INTO tab(id, col) VALUES (1,'a'), (2, 'b');
INSERT 0 2
so=# INSERT INTO tab(id, col)
VALUES (3, 'c'), (4, 'd'), (1,'aaaa')
ON CONFLICT (id) DO UPDATE SET col = EXCLUDED.col
returning *,case when xmax::text::int > 0 then 'updated' else 'inserted' end,ctid;
 id | col  |   case   | ctid
----+------+----------+-------
  3 | c    | inserted | (0,3)
  4 | d    | inserted | (0,4)
  1 | aaaa | updated  | (0,5)
(3 rows)

INSERT 0 3
so=# INSERT INTO tab(id, col)
VALUES (3, 'c'), (4, 'd'), (1,'aaaa')
ON CONFLICT (id) DO UPDATE SET col = EXCLUDED.col
returning *,case when xmax::text::int > 0 then 'updated' else 'inserted' end,ctid;
 id | col  |  case   | ctid
----+------+---------+-------
  3 | c    | updated | (0,6)
  4 | d    | updated | (0,7)
  1 | aaaa | updated | (0,8)
(3 rows)

INSERT 0 3

Answer 4

(xmax::text::bigint > 0) 或 (NOT xmax = 0)。一旦交易计数达到整数溢出，类型转换为整数就会中断。

如何确定 upsert 是否是 PostgreSQL 9.5+ UPSERT 的更新？

How to find out if an upsert was an update with PostgreSQL 9.5+ UPSERT?

postgresql

upsert

postgresql-9.5