现象

记录一个生产问题，DBA在执行下面这个GRANT操作的时候，偶发的会出现这个报错

[guqi@intel175 ~]$ psql -p 51300
psql (14devel)
Type "help" for help.

guqi=# grant select on all tables in schema public to guqi ;
ERROR:  tuple concurrently updated
guqi=# \q

分析

GRANT操作不会更新表内的数据，应该只是会更新系统表内的行。换言之，当执行GRANT的时候，有些数据表在系统表内的行被别人更新了。设置下errverbose，发现是在更新pg_class的relacl属性

[guqi@intel175 ~]$ psql -p 51300
psql (14devel)
Type "help" for help.
guqi=#\errverbose
guqi=# grant select on all tables in schema public to guqi ;
^CCancel request sent
ERROR:  canceling statement due to user request
CONTEXT:  while updating tuple (6,6) in relation "pg_class"
guqi=# select * from pg_class where ctid = '(6,6)';
  oid  | relname | relnamespace | reltype | reloftype | relowner | relam | relfilenode | reltablespace | relpages | reltuples
| relallvisible | reltoastrelid | relhasindex | relisshared | relpersistence | relkind | relnatts | relchecks | relhasrules |
relhastriggers | relhassubclass | relrowsecurity | relforcerowsecurity | relispopulated | relreplident | relispartition | relr
ewrite | relfrozenxid | relminmxid | relacl | reloptions | relpartbound
-------+---------+--------------+---------+-----------+----------+-------+-------------+---------------+----------+-----------
+---------------+---------------+-------------+-------------+----------------+---------+----------+-----------+-------------+-
---------------+----------------+----------------+---------------------+----------------+--------------+----------------+-----
-------+--------------+------------+--------+------------+--------------
 16458 | tb1     |         2200 |   16460 |         0 |       10 |     2 |       16462 |             0 |        0 |        -1
|             0 |             0 | f           | f           | p              | r       |        2 |         0 | f           |
f              | f              | f              | f                   | t              | d            | f              |
     0 |          580 |          1 |        |            |
(1 row)

同时查看pg_stat_activity，发现有TRUNCATE在运行，与内核开发的小伙伴沟通了下，truncate在内部会重建表，重建会导致表的物理存储发生变化。基于这一点，我测试了下

# 窗口1
guqi=# begin;
BEGIN
guqi=*# truncate tb1 ;
TRUNCATE TABLE
-- 暂不提交

# 窗口2
guqi=# grant select on all tables in schema public to guqi ;
-- 操作会处于锁等状态

# 窗口1
guqi=*# commit;
COMMIT

# 窗口2
guqi=# grant select on all tables in schema public to guqi ;
ERROR:  tuple concurrently updated

按照上述顺序执行，100%触发，而且通过跟踪tb1在pg_class内的ctid值，发现TRUNCATE前后确实变了，那么基本原因就是这个了。

解决的话，把GRANT和TRUNCATE分开执行，不要并发。