


How to Handle Missing Rows in PostgreSQL UPSERTs with Concurrent Transactions?
Handling missing result rows in PostgreSQL UPSERT operation
In PostgreSQL 9.5, using RETURNING
and ON CONFLICT
together sometimes resulted in missing rows when concurrent transactions updated specified conflicting targets.
Disadvantages of the currently accepted answer
The currently accepted answer is prone to problems when concurrent transactions update multiple rows in the target table. While it prevents missing rows, it introduces other side effects and performance penalties associated with empty updates.
Alternatives
No concurrent write load:
WITH input_rows(usr, contact, name) AS ( VALUES (text 'foo1', text 'bar1', text 'bob1') -- 第一行中的类型转换 , ('foo2', 'bar2', 'bob2') -- 更多? ) , ins AS ( INSERT INTO chats (usr, contact, name) SELECT * FROM input_rows ON CONFLICT (usr, contact) DO NOTHING RETURNING id --, usr, contact -- 返回更多列? ) SELECT 'i' AS source -- 'i' 代表'插入' , id --, usr, contact -- 返回更多列? FROM ins UNION ALL SELECT 's' AS source -- 's' 代表'选择' , c.id --, usr, contact -- 返回更多列? FROM input_rows JOIN chats c USING (usr, contact); -- 唯一索引的列
Having concurrent write load
To resolve race conditions for multiple concurrent transactions:
WITH input_rows(usr, contact, name) AS ( ... ) -- 如上所示 , ins AS ( INSERT INTO chats AS c (usr, contact, name) SELECT * FROM input_rows ON CONFLICT (usr, contact) DO NOTHING RETURNING id, usr, contact -- 我们需要唯一的列来进行后续连接 ) , sel AS ( SELECT 'i'::"char" AS source -- 'i' 代表'插入' , id, usr, contact FROM ins UNION ALL SELECT 's'::"char" AS source -- 's' 代表'选择' , c.id, usr, contact FROM input_rows JOIN chats c USING (usr, contact) ) , ups AS ( -- 罕见的极端情况 INSERT INTO chats AS c (usr, contact, name) -- 另一个UPSERT,不仅仅是UPDATE SELECT i.* FROM input_rows i LEFT JOIN sel s USING (usr, contact) -- 唯一索引的列 WHERE s.usr IS NULL -- 缺失! ON CONFLICT (usr, contact) DO UPDATE -- 我们第一次已经礼貌地请求了…… SET name = c.name -- ……这次我们用旧值覆盖 RETURNING 'u'::"char" AS source -- 'u' 代表更新 , id --, usr, contact -- 返回更多列? ) SELECT source, id FROM sel UNION ALL TABLE ups;
Key points:
-
sel
CTE returns inserted rows and selected rows, providing the complete output result set. -
ups
CTE handles missing rows, updating them with existing values. - The output result set excludes any rows updated by concurrent transactions unless they are
ups
processed.
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