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这篇文章主要讲解了“PostgreSQL 的 pg_promote 有什么作用”，文中的讲解内容简单清晰，易于学习与理解，下面请大家跟着丸趣 TV 小编的思路慢慢深入，一起来研究和学习“PostgreSQL 的 pg_promote 有什么作用”吧！
在 PG 12 以前的版本, 备库提升为主库需在备库主机上执行命令或者通过生成触发文件进行触发, 在 PG 12 中, 可通过客户端连接到数据库后执行 pg_promote 函数实现.
下面以一个简单的例子进行说明.
搭建流复制环境
参照
PostgreSQL DBA(31) – Backup Recovery#4(搭建流复制)
, 注意在 PG 12,recovery.conf 文件已废弃, 相关的配置信息已合并至 postgresql.conf 文件中.
下面是搭建完毕后, 在主库创建数据表 t1 后, 备库的情况.
备库
[pg12@localhost pg12db1]$ pg_ctl start
waiting for server to start....2019-06-20 17:04:04.361 CST [23158] LOG: starting PostgreSQL 12beta1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16), 64-bit
2019-06-20 17:04:04.362 CST [23158] LOG: listening on IPv4 address 0.0.0.0 , port 5432
2019-06-20 17:04:04.362 CST [23158] LOG: listening on IPv6 address :: , port 5432
2019-06-20 17:04:04.365 CST [23158] LOG: listening on Unix socket /tmp/.s.PGSQL.5432
2019-06-20 17:04:04.422 CST [23158] LOG: redirecting log output to logging collector process
2019-06-20 17:04:04.422 CST [23158] HINT: Future log output will appear in directory pg_log .
done
server started
[pg12@localhost pg12db1]$ psql -d testdb
psql (12beta1)
Type help for help.
testdb=# select count(*) from t1;
psql: ERROR: relation t1 does not exist
LINE 1: select count(*) from t1;
^
testdb=# select count(*) from t1;
count
-------
0
(1 row)下面通过远程客户端连接到备库, 执行函数 pg_promote 提升备库为主库.
[pg12@localhost pg12db1]$ psql -h 192.168.26.27 -U pg12 -d testdb
psql (12beta1)
Type help for help.
testdb=# -- Wait for at most 30 seconds.
testdb=# SELECT pg_promote(true, 30);
pg_promote
------------
t
(1 row)日志输出
,,,,, SELECT pg_promote(true, 30); ,,, psql
2019-06-20 17:14:22.584 CST,,,23160,,5d0b4c04.5a78,5,,2019-06-20 17:04:04 CST,1/0,0,LOG,00000, received promote request ,,,,,,,,,
2019-06-20 17:14:22.584 CST,,,23167,,5d0b4c04.5a7f,2,,2019-06-20 17:04:04 CST,,0,FATAL,57P01, terminating walreceiver process due to administrator command ,,,,,,,,,
2019-06-20 17:14:22.625 CST,,,23160,,5d0b4c04.5a78,6,,2019-06-20 17:04:04 CST,1/0,0,LOG,00000, invalid record length at 0/5016D48: wanted 24, got 0 ,,,,,,,,,
2019-06-20 17:14:22.625 CST,,,23160,,5d0b4c04.5a78,7,,2019-06-20 17:04:04 CST,1/0,0,LOG,00000, redo done at 0/5016D10 ,,,,,,,,,
2019-06-20 17:14:22.625 CST,,,23160,,5d0b4c04.5a78,8,,2019-06-20 17:04:04 CST,1/0,0,LOG,00000, last completed transaction was at log time 2019-06-20 17:04:49.180746+08 ,,,,,,,,,
2019-06-20 17:14:22.649 CST,,,23160,,5d0b4c04.5a78,9,,2019-06-20 17:04:04 CST,1/0,0,LOG,00000, selected new timeline ID: 2 ,,,,,,,,,
2019-06-20 17:14:22.738 CST,,,23160,,5d0b4c04.5a78,10,,2019-06-20 17:04:04 CST,1/0,0,LOG,00000, archive recovery complete ,,,,,,,,,
2019-06-20 17:14:22.755 CST,,,23158,,5d0b4c04.5a76,3,,2019-06-20 17:04:04 CST,,0,LOG,00000, database system is ready to accept connections ,,,,,,,,,
2019-06-20 17:14:22.764 CST,,,23277,,5d0b4e6e.5aed,1,,2019-06-20 17:14:22 CST,,0,FATAL,XX000, archive command failed with exit code 127 , The failed archive command was: /home/pg12/archive.sh ,,,,,,,,
2019-06-20 17:14:22.766 CST,,,23158,,5d0b4c04.5a76,4,,2019-06-20 17:04:04 CST,,0,LOG,00000, archiver process (PID 23277) exited with exit code 1 ,,,,,,,,,
2019-06-20 17:15:22.779 CST,,,23329,,5d0b4eaa.5b21,1,,2019-06-20 17:15:22 CST,,0,FATAL,XX000, archive command failed with exit code 127 , The failed archive command was: /home/pg12/archive.sh ,,,,,,,,
2019-06-20 17:15:22.781 CST,,,23158,,5d0b4c04.5a76,5,,2019-06-20 17:04:04 CST,,0,LOG,00000, archiver process (PID 23329) exited with exit code 1 ,,,,,,,,,
[pg12@localhost pg_log]$pg_control 中的状态
[pg12@localhost pg12db1]$ pg_controldata|grep state
Database cluster state: in production从日志 状态可看出, 备库已提升为主库(出错提示是没有找到归档命令).
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