PostgreSQL数据库里有多个和会话相关的参数，PostgreSQL17-beta1版本新增了一个transaction_timeout参数，来限制事务的持续时间。
当前的一些和会话相关的超时参数如下
+-------------------------------------+---------+------+-------------------------------------------------+---------+---------+---------+------------+----------+
| name | setting | unit | category | context | vartype | min_val | max_val | enumvals |
+-------------------------------------+---------+------+-------------------------------------------------+---------+---------+---------+------------+----------+
| idle_in_transaction_session_timeout | 0 | ms | Client Connection Defaults / Statement Behavior | user | integer | 0 | 2147483647 | |
| idle_session_timeout | 0 | ms | Client Connection Defaults / Statement Behavior | user | integer | 0 | 2147483647 | |
| lock_timeout | 0 | ms | Client Connection Defaults / Statement Behavior | user | integer | 0 | 2147483647 | |
| statement_timeout | 0 | ms | Client Connection Defaults / Statement Behavior | user | integer | 0 | 2147483647 | |
| transaction_timeout | 0 | ms | Client Connection Defaults / Statement Behavior | user | integer | 0 | 2147483647 | |
+-------------------------------------+---------+------+-------------------------------------------------+---------+---------+---------+------------+----------+
(5 rows)
| 参数名 | 作用 |
|---|---|
| idle_in_transaction_session_timeout | IDLE IN TRANSACTION状态会话超时时间 |
| idle_session_timeout | IDLE 状态会话超时时间 |
| lock_timeout | 锁等待超时时间 |
| statement_timeout | 一条SQL语句执行的超时时间 |
| transaction_timeout | 一个事务执行的超时时间 |
一、关于transaction_timeout参数
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postgres<17beta1>(ConnAs[postgres]:PID[21590] 2024-05-28/17:08:02)=# select * from pg_settings where name like 'transaction_timeout';
+-[ RECORD 1 ]----+-----------------------------------------------------------------------------------------------------+
| name | transaction_timeout |
| setting | 0 |
| unit | ms |
| category | Client Connection Defaults / Statement Behavior |
| short_desc | Sets the maximum allowed duration of any transaction within a session (not a prepared transaction). |
| extra_desc | A value of 0 turns off the timeout. |
| context | user |
| vartype | integer |
| source | default |
| min_val | 0 |
| max_val | 2147483647 |
| enumvals | |
| boot_val | 0 |
| reset_val | 0 |
| sourcefile | |
| sourceline | |
| pending_restart | f |
+-----------------+-----------------------------------------------------------------------------------------------------+
1.相关注意
如果transaction_timeout值为0，则认为超时控制无效。如果transaction_timeout小于或等于idle_in_transaction_session_timeout或statement_timeout，则忽略较长的超时。
2.使用场景
可以根据实际场景合理配置，一定程度避免长事务。
比如一个事务内发出许多低延迟的查询，但是一个事务内执行的SQL数量很大的时候，整个事务需要很长时间处理。可以通过该参数让这个长的事务回滚。
二、transaction_timeout参数效果测试
首先编辑一个SQL文件，编辑test_transaction_timeout.sql
内容如下：
begin;
\timing
set statement_timeout = 1500;
--set transaction_timeout=3000;
select 'ysla tests-1';
select pg_sleep(1);
select 'ysla tests-2';
select pg_sleep(1);
select 'ysla tests-3';
select pg_sleep(1);
select 'ysla tests-4';
commit;
在一个事务里重复执行打印字符以及pg_sleep(1)函数,这两部分分别当作低延迟的查询。然后在事务的开头设置statement_timeout = 1500。transaction_timeout参数暂时先注释掉。事务中的任何 SQL 语句都不会花费超过 1500 毫秒的时间，因此事务将运行完成。
下面跑一下结果，可以看到未触发statement_timeout超时，事务正常结束了。
[postgres@xmaster-PostgreSQL-17beta1-06 ~]$ psql -f test_transaction_timeout.sql
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BEGIN
Timing is on.
SET
Time: 0.168 ms
+--------------+
| ?column? |
+--------------+
| ysla tests-1 |
+--------------+
(1 row)Time: 0.298 ms
+----------+
| pg_sleep |
+----------+
| |
+----------+
(1 row)Time: 1013.626 ms (00:01.014)
+--------------+
| ?column? |
+--------------+
| ysla tests-2 |
+--------------+
(1 row)Time: 0.334 ms
+----------+
| pg_sleep |
+----------+
| |
+----------+
(1 row)Time: 1014.618 ms (00:01.015)
+--------------+
| ?column? |
+--------------+
| ysla tests-3 |
+--------------+
(1 row)Time: 0.741 ms
+----------+
| pg_sleep |
+----------+
| |
+----------+
(1 row)Time: 1014.584 ms (00:01.015)
+--------------+
| ?column? |
+--------------+
| ysla tests-4 |
+--------------+
(1 row)Time: 0.343 ms
COMMIT
Time: 0.525 ms
然后把SQL文件里的transaction_timeout参数的注释打开，此时，statement_timeout=1500，事务内的任何 SQL 语句都不会超过 1500ms，但第三次pg_sleep(1)执行的时候将超过transaction_timeout设置的3s阈值，导致超时错误，会话将被终止，事务也被终止。可以看到打印的信息里也显示了： FATAL: terminating connection due to transaction timeout
表示是由于触发了transaction_timeout参数的阈值而引发的连接断开。
[postgres@xmaster-PostgreSQL-17beta1-06 ~]$ psql -f test_transaction_timeout.sql
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BEGIN
Timing is on.
SET
Time: 0.185 ms
SET
Time: 0.090 ms
+--------------+
| ?column? |
+--------------+
| ysla tests-1 |
+--------------+
(1 row)Time: 0.520 ms
+----------+
| pg_sleep |
+----------+
| |
+----------+
(1 row)Time: 1000.747 ms (00:01.001)
+--------------+
| ?column? |
+--------------+
| ysla tests-2 |
+--------------+
(1 row)Time: 0.365 ms
+----------+
| pg_sleep |
+----------+
| |
+----------+
(1 row)Time: 1004.818 ms (00:01.005)
+--------------+
| ?column? |
+--------------+
| ysla tests-3 |
+--------------+
(1 row)Time: 0.440 ms
psql:test_transaction_timeout.sql:10: FATAL: terminating connection due to transaction timeout
psql:test_transaction_timeout.sql:10: server closed the connection unexpectedlyThis probably means the server terminated abnormallybefore or while processing the request.
psql:test_transaction_timeout.sql:10: error: connection to server was lost
所以，PostgreSQL17-beta1新增的这个务级别超时参数transaction_timeout，在结合我们实际场景合理配置的情况下，可以一定程度避免长事务，避免数据库里存在：单个SQL执行时间短，但是单个事务里包含的SQL较多导致的长事务。需要额外提一嘴的是，如果实际业务就是做的一个事务里包含大量的低延迟SQL，那最好是和业务沟通是否可以进行优化拆分，毕竟参数只是限制时长，达到阈值让他回滚，避免数据库里的事务长时间存在。但是不能从源头上阻止此类长事务的产生。
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