如何查看upsert是否是PostgreSQL 9.5+ UPSERT的更新？

Question

如Insert, on duplicate update in PostgreSQL?所述，可写CTE在9.5之前被认为是UPSERT的解决方案

可以执行带有以下信息的UPSERT，无论它是以UPDATE还是INSERT与以下可写CTE惯用语：

WITH
    update_cte AS (
        UPDATE t SET v = $1 WHERE id = $2 RETURNING 'updated'::text status
    ),
    insert_cte AS (
        INSERT INTO t(id, v) SELECT $2, $1 WHERE NOT EXISTS
            (SELECT 1 FROM update_cte) RETURNING 'inserted'::text status
    )
 (SELECT status FROM update_cte) UNION (SELECT status FROM insert_cte)

此查询将返回“已更新”或“已插入”，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

使用PostgreSQL 9.5+新的“UPSERT”语法可以实现类似的功能，从优化中获益并避免可能的约束违规吗？

Answer 1

我相信xmax::text::int > 0将是最简单的技巧：

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 2

从@lad2025's answer绘制，结果可以通过在WHERE子句中滥用settings和customized options与related functions来实现，以获得所需的副作用。

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

INSERT INTO t (id, v)
    SELECT $1, $2
    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意味着设置将在交易结束时消失。更确切地说，current_setting('upsert.action')将返回NULL（并且不会抛出错误）直到会话结束。

Answer 3

在SQL Server MERGE声明有$action返回字符串'INSERT', 'UPDATE', or 'DELETE'。

对于qazxsw poi，我找不到与qazxsw poi类似的函数/变量。

解决方法的一种方法是将Postgresql列添加到表中：

RETURNING

is_updated

输出：

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;

Answer 4

Rextester Demo或

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

。一旦事务计数达到整数溢出，Typecast到整数就会中断。