postgresql: prepare transaction

普通事务为session级别，prepare transaction将当前事务与当前session分离，进行持久化记录，带来的好处是提高事务提交成功概率。

普通事务是session级别的，在commit过程中，如果server发生异常，如系统崩溃，断电等，session消失，commit失败。而prepare transaction是持久化记录的，即使发生系统崩溃，重启后事务仍然可以进行commit。

prepare transaction主要用于实现两阶段提交，一般用于事务管理器使用，普通client不要直接使用，应该使用begin， start transaction。

前提：系统参数max_prepared_transactions需要大于0
设置方法见alter system

如果max_prepared_transactions为0会报错

postgres=# prepare transaction 'prep1';
ERROR:  prepared transactions are disabled
HINT:  Set max_prepared_transactions to a nonzero value.

prepare transaction
特别的，执行prepare transaction后，原来的事务的作用域立刻结束了，查询事务中insert的数据也不存在。此时也可以再begin新的事务。

postgres=# begin;
BEGIN
postgres=# insert into t1 values(666, 'test666');
INSERT 0 1
postgres=# select * from t1 where id=666;
 id  |  name
-----+---------
 666 | test666
(1 row)

postgres=# prepare transaction 'prep1';
PREPARE TRANSACTION

postgres=# select * from t1 where id=666;
 id | name
----+------
(0 rows)


postgres=# select * from pg_prepared_xacts;
 transaction |  gid  |           prepared            |  owner   | database
-------------+-------+-------------------------------+----------+----------
        3316 | prep1 | 2021-06-25 01:48:39.618381+00 | postgres | postgres
(1 row)

postgres=# \q

commit prepared
可以在prepare transaction的session中执行，也可在其它session中执行，也可以在server重启后执行。

[postgres@hgcndn ~]$ psql -p 5432 -U postgres -h 127.0.0.1
psql (12.4)
Type "help" for help.

postgres=#  select * from pg_prepared_xacts;
 transaction |  gid  |           prepared            |  owner   | database
-------------+-------+-------------------------------+----------+----------
        3316 | prep1 | 2021-06-25 01:48:39.618381+00 | postgres | postgres
(1 row)

postgres=# select * from t1 where id=666;
 id | name
----+------
(0 rows)

postgres=# commit prepared 'prep1';
COMMIT PREPARED

postgres=# select * from t1 where id=666;
 id  |  name
-----+---------
 666 | test666
(1 row)

postgres=# select * from pg_prepared_xacts;
 transaction | gid | prepared | owner | database
-------------+-----+----------+-------+----------
(0 rows)

rollback prepared
可以在prepare transaction的session中执行，也可在其它session中执行，也可以在server重启后执行。

postgres=# begin;
BEGIN
postgres=# insert into t1 values(777, 'test777');
INSERT 0 1
postgres=# prepare transaction 'prep2';
PREPARE TRANSACTION
postgres=#  select * from pg_prepared_xacts;
 transaction |  gid  |           prepared            |  owner   | database
-------------+-------+-------------------------------+----------+----------
        3317 | prep2 | 2021-06-25 02:21:52.226903+00 | postgres | postgres
(1 row)

postgres=#
postgres=# rollback prepared 'prep2';
ROLLBACK PREPARED
postgres=#
postgres=#  select * from pg_prepared_xacts;
 transaction | gid | prepared | owner | database
-------------+-----+----------+-------+----------
(0 rows)

特别说明，对于事务中set(非local的情况)，set后再prepare transaction后，set的参数会继续生效。引用pg官方文档的说明，“Thus, in this one respect PREPARE TRANSACTION acts more like COMMIT than ROLLBACK.”

postgres=# show client_encoding;
 client_encoding
-----------------
 UTF8
(1 row)

postgres=# begin;
BEGIN
postgres=# set client_encoding  to 'UNICODE';
SET
postgres=# show client_encoding;
 client_encoding
-----------------
 UNICODE
(1 row)

postgres=# prepare transaction 'prep1';
PREPARE TRANSACTION
postgres=# show client_encoding;
 client_encoding
-----------------
 UNICODE
(1 row)

参考资料
https://www.postgresql.org/docs/13/sql-prepare-transaction.html
https://www.postgresql.org/docs/13/sql-commit-prepared.html
https://www.postgresql.org/docs/13/sql-rollback-prepared.html