本文簡單介紹了PostgreSQL的外掛：pg_cron。該外掛可在PG中實現類似crontab的功能。

**安裝**
編譯安裝
```
[pg12@localhost pg_cron]$git clone https://github.com/citusdata/pg_cron.git
[pg12@localhost pg_cron]$cd pg_cron
[pg12@localhost pg_cron]$ make
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-implicit-fallthrough -Iinclude -I/appdb/pg12/pg12.1/include -I. -I./ -I/appdb/pg12/pg12.1/include/postgresql/server -I/appdb/pg12/pg12.1/include/postgresql/internal  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o src/pg_cron.o src/pg_cron.c -MMD -MP -MF .deps/pg_cron.Po
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-implicit-fallthrough -Iinclude -I/appdb/pg12/pg12.1/include -I. -I./ -I/appdb/pg12/pg12.1/include/postgresql/server -I/appdb/pg12/pg12.1/include/postgresql/internal  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o src/job_metadata.o src/job_metadata.c -MMD -MP -MF .deps/job_metadata.Po
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-implicit-fallthrough -Iinclude -I/appdb/pg12/pg12.1/include -I. -I./ -I/appdb/pg12/pg12.1/include/postgresql/server -I/appdb/pg12/pg12.1/include/postgresql/internal  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o src/misc.o src/misc.c -MMD -MP -MF .deps/misc.Po
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-implicit-fallthrough -Iinclude -I/appdb/pg12/pg12.1/include -I. -I./ -I/appdb/pg12/pg12.1/include/postgresql/server -I/appdb/pg12/pg12.1/include/postgresql/internal  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o src/task_states.o src/task_states.c -MMD -MP -MF .deps/task_states.Po
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-implicit-fallthrough -Iinclude -I/appdb/pg12/pg12.1/include -I. -I./ -I/appdb/pg12/pg12.1/include/postgresql/server -I/appdb/pg12/pg12.1/include/postgresql/internal  -D_GNU_SOURCE -I/usr/include/libxml2   -c -o src/entry.o src/entry.c -MMD -MP -MF .deps/entry.Po
gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -O0 -DOPTIMIZER_DEBUG -g3 -gdwarf-2 -fPIC -shared -o pg_cron.so src/pg_cron.o src/job_metadata.o src/misc.o src/task_states.o src/entry.o -L/appdb/pg12/pg12.1/lib    -Wl,--as-needed -Wl,-rpath,'/appdb/pg12/pg12.1/lib',--enable-new-dtags  -L/appdb/pg12/pg12.1/lib -lpq 
cat pg_cron.sql > pg_cron--1.0.sql
[pg12@localhost pg_cron]$ make install
/bin/mkdir -p '/appdb/pg12/pg12.1/lib/postgresql'
/bin/mkdir -p '/appdb/pg12/pg12.1/share/postgresql/extension'
/bin/mkdir -p '/appdb/pg12/pg12.1/share/postgresql/extension'
/bin/install -c -m 755  pg_cron.so '/appdb/pg12/pg12.1/lib/postgresql/pg_cron.so'
/bin/install -c -m 644 .//pg_cron.control '/appdb/pg12/pg12.1/share/postgresql/extension/'
/bin/install -c -m 644 .//pg_cron--1.0--1.1.sql .//pg_cron--1.1--1.2.sql pg_cron--1.0.sql '/appdb/pg12/pg12.1/share/postgresql/extension/'
[pg12@localhost pg_cron]$

```

**體驗**
建立擴充套件
```
[local:/data/run/pg12]:5120 pg12@testdb=# create extension pg_cron;
ERROR:  can only create extension in database postgres
DETAIL:  Jobs must be scheduled from the database configured in cron.database_name, since the pg_cron background worker reads job descriptions from this database.
HINT:  Add cron.database_name = 'testdb' in postgresql.conf to use the current database.
CONTEXT:  PL/pgSQL function inline_code_block line 4 at RAISE
[local:/data/run/pg12]:5120 pg12@testdb=# 
[local:/data/run/pg12]:5120 pg12@testdb=# \c postgres
You are now connected to database "postgres" as user "pg12".
[local:/data/run/pg12]:5120 pg12@postgres=# create extension pg_cron;
CREATE EXTENSION
[local:/data/run/pg12]:5120 pg12@postgres=#

```
修改系統引數
```
[local:/data/run/pg12]:5120 pg12@postgres=# select name,setting from pg_settings where name like '%cron%';
         name          |  setting  
-----------------------+-----------
 cron.database_name    | postgres
 cron.host             | localhost
 cron.log_statement    | on
 cron.max_running_jobs | 32
(4 rows)

[local:/data/run/pg12]:5120 pg12@postgres=#

```
建立job，目的是每個1分鐘執行VACUUM FULL
```
[local:/data/run/pg12]:5120 pg12@postgres=# SELECT cron.schedule('*/1 * * * *', 'VACUUM FULL');
 schedule 
----------
        1
(1 row)

```

日誌輸出
```
2020-02-11 17:52:59.935 CST,"pg12","postgres",23764,"[local]",5e427932.5cd4,1,"SELECT",2020-02-11 17:51:46 CST,3/8,0,LOG,00000,"failed to parse entry 1",,,,,,"SELECT cron.schedule('0/1 * * * *', 'VACUUM FULL');",,,"psql"
2020-02-11 17:52:59.936 CST,"pg12","postgres",23764,"[local]",5e427932.5cd4,2,"SELECT",2020-02-11 17:51:46 CST,3/8,0,ERROR,22023,"invalid schedule: 0/1 * * * *",,,,,,"SELECT cron.schedule('0/1 * * * *', 'VACUUM FULL');",,,"psql"
2020-02-11 17:54:00.001 CST,,,19164,,5e426547.4adc,2,,2020-02-11 16:26:47 CST,1/0,0,LOG,00000,"cron job 1 starting: VACUUM FULL",,,,,,,,,""
2020-02-11 17:54:00.740 CST,,,19164,,5e426547.4adc,3,,2020-02-11 16:26:47 CST,1/0,0,LOG,00000,"cron job 1 completed: VACUUM ",,,,,,,,,""
2020-02-11 17:55:00.002 CST,,,19164,,5e426547.4adc,4,,2020-02-11 16:26:47 CST,1/0,0,LOG,00000,"cron job 1 starting: VACUUM FULL",,,,,,,,,""
2020-02-11 17:55:00.579 CST,,,19164,,5e426547.4adc,5,,2020-02-11 16:26:47 CST,1/0,0,LOG,00000,"cron job 1 completed: VACUUM ",,,,,,,,,""

```
取消排程
```
[local:/data/run/pg12]:5120 pg12@postgres=# select cron.unschedule(1);
 unschedule 
------------
 t
(1 row)

[local:/data/run/pg12]:5120 pg12@postgres=#

```

**參考資料**
[pg_cron](https://github.com/citusdata/pg_cron)

PostgreSQL DBA(163) - Extension(pg_cron)

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