Question

我有一个查询只有在外部引用（用户，会话）已经存在时才在数据库中插入事件。

如果用户不存在，则会插入并保存ID
如果会话不存在，则插入前一个来自用户的ID
使用先前创建的userid和sessions id插入事件。

。我收到此错误：

“PHP警告：pg_query_params（）：查询失败：错误：语法错误在“IF”处或附近

此查询：

BEGIN;
SELECT id FROM users WHERE mongoid = $1 (userid); 
IF not found THEN
    -- inserts user and should remember userid
    INSERT INTO users (mongoid, shopid, idinshop, attributes)
        VALUES ($1, $2, $3, $4) RETURNING id (userid);
END IF;
--looks for session and should remember sessionid
SELECT id FROM sessions WHERE mongoid = $5 (sessionid);
IF not found THEN
     -- inserts session
     INSERT INTO sessions (mongoid, shopid, userid, session, traffic, counts)
               VALUES ($5, $2, (userid), $6, $7, $8) RETURNING id (sessionid);
END IF;
-- finally inserts the event
INSERT INTO events (shopid, sessionid, userid, type, attributes, mongoid) 
     VALUES ($2, (sessionid), (userid), $9, $10, $11);
COMMIT;

稍后编辑：我使用下面的答案解决了这个问题！

Answer 1

您可以使用data-modifying CTEs在单 SQL语句中使用纯SQL 重写过程逻辑。

WITH usr1 AS (SELECT id FROM users WHERE mongoid = $1)
   , usr2 AS (
      INSERT INTO users (mongoid, shopid, idinshop, attributes)
      SELECT $1, $2, $3, $4
      WHERE  NOT EXISTS (SELECT 1 FROM usr1)
      RETURNING id
      )
   , ses1 AS (SELECT id FROM sessions WHERE mongoid = $5)
   , ses2 AS (
      INSERT INTO sessions (mongoid, shopid, userid, session, traffic, counts)
      SELECT $5, $2, (SELECT id FROM usr1 NATURAL FULL OUTER JOIN usr2
           , $6, $7, $8
      WHERE  NOT EXISTS (SELECT 1 FROM ses1)
      RETURNING id
      )
INSERT INTO events (shopid, sessionid, userid, type, attributes, mongoid) 
VALUES ($2
     , (SELECT id FROM usr1 NATURAL FULL OUTER JOIN usr2)
     , (SELECT id FROM ses1 NATURAL FULL OUTER JOIN ses2)
     , $9, $10, $11);

需要Postgres 9.1 或更高版本未经测试。如果您想要测试答案，请在您的问题中提供测试用例。

应该比单个命令（let alone repeated round trips to the db server!）快得多。

请注意并发负载过重的潜在并发问题。提出的单个陈述已经很多不太可能导致麻烦。但可能性存在。可能的解决方案包括manual locking（昂贵）advisory locks或serializable transactions（也可能很贵）。

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