PostgreSQL 9.6
ALTER TABLE "Users"
ADD COLUMN "someField" BOOLEAN NULL DEFAULT NULL;
"Users"
有 5,000 行,但数据库非常大(150+ Gb)。 SELECT
用户表上的查询出现,(检查):SELECT query FROM pg_stat_activity
WHERE state = 'active' and query LIKE 'SELECT%'
(之前,没有查询)
SELECT
查询占用所有 CPU。 VACUUM ANALYZE
在 "Users"
表:INFO: vacuuming "public.Users"
INFO: index "Users_pkey" now contains 5556 row versions in 86 pages
DETAIL: 0 index row versions were removed.
1 index pages have been deleted, 1 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.12 sec.
INFO: index "users_subscription_canceled" now contains 5028 row versions in 508 pages
DETAIL: 0 index row versions were removed.
8 index pages have been deleted, 8 are currently reusable.
CPU 0.00s/0.00u sec elapsed 1.54 sec.
INFO: index "users_shopper_id" now contains 5556 row versions in 205 pages
DETAIL: 0 index row versions were removed.
1 index pages have been deleted, 1 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.64 sec.
INFO: index "users_referrer" now contains 5556 row versions in 586 pages
DETAIL: 0 index row versions were removed.
4 index pages have been deleted, 4 are currently reusable.
CPU 0.00s/0.00u sec elapsed 1.80 sec.
INFO: index "users_referral_code" now contains 5556 row versions in 84 pages
DETAIL: 0 index row versions were removed.
2 index pages have been deleted, 2 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.27 sec.
INFO: "Users": found 0 removable, 2137 nonremovable row versions in 803 out of 2780 pages
DETAIL: 445 dead row versions cannot be removed yet.
There were 25647 unused item pointers.
Skipped 0 pages due to buffer pins.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 6.91 sec.
INFO: "Users": stopping truncate due to conflicting lock request
INFO: vacuuming "pg_toast.pg_toast_26460"
INFO: index "pg_toast_26460_index" now contains 0 row versions in 1 pages
DETAIL: 0 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_toast_26460": found 0 removable, 0 nonremovable row versions in 0 out of 0 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
Skipped 0 pages due to buffer pins.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: analyzing "public.Users"
INFO: "Users": scanned 2780 of 2780 pages, containing 5539 live rows and 459 dead rows; 5539 rows in sample, 5539 estimated total rows
VACUUM
DROP COLUMN
或 RENAME COLUMN
,同样的第 4 点和第 5 点发生 - 并且数据库卡住。 问题:
最佳答案
这是正常行为;问题是你的工作量。
你说得对ALTER TABLE ... ADD COLUMN
是一个非常快的操作。那不是问题。问题是这样的ALTER TABLE
需要短ACCESS EXCLUSIVE
锁定表,因为它修改了表结构。
这样的ACCESS EXCLUSIVE
锁与 ACCESS SHARE
不兼容锁定一个 SELECT
声明放在 table 上。这就是目的:应该如何SELECT
如果表在运行时发生更改,则语句的行为?
现在的问题是您的查询需要很长时间,或者有人忘记关闭对表有锁定的事务。
你可以检查这个
SELECT pid, a.state, a.xact_start
FROM pg_locks AS l
JOIN pg_stat_activity AS a USING (pid)
WHERE l.relation = 'Users'::regclass;
这将显示所有在表上有锁定的事务以及它们何时开始。
现在您的
ALTER TABLE
必须等到所有这些交易完成,以及所有的短SELECT
稍后发出的语句必须排在 ALTER TABLE
后面。 .尽快
ALTER TABLE
获得它需要的锁,它会很快完成并释放表上的锁。现在,所有其他排队的语句将同时松散,并在您的机器上造成高负载。解决方案由两部分组成:
max_connections
尽可能使用连接池。那么可以被阻塞的语句数量是有限的,机器过载的危险就更小了。 关于Postgresql Alter Table 卡住 DB(CPU 被某些 Select 高负载),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/61137550/