base on Run periodic jobs in PostgreSQL [](https://www.citusdata.com/)
## What is pg_cron?
pg_cron is a simple cron-based job scheduler for PostgreSQL (10 or higher) that runs inside the database as an extension. It uses the same syntax as regular cron, but it allows you to schedule PostgreSQL commands directly from the database. You can also use '[1-59] seconds' to schedule a job based on an interval.
pg_cron also allows you using '$' to indicate last day of the month.
```sql
-- Delete old data on Saturday at 3:30am (GMT)
SELECT cron.schedule('30 3 * * 6', $$DELETE FROM events WHERE event_time < now() - interval '1 week'$$);
schedule
----------
42
-- Vacuum every day at 10:00am (GMT)
SELECT cron.schedule('nightly-vacuum', '0 10 * * *', 'VACUUM');
schedule
----------
43
-- Change to vacuum at 3:00am (GMT)
SELECT cron.schedule('nightly-vacuum', '0 3 * * *', 'VACUUM');
schedule
----------
43
-- Stop scheduling jobs
SELECT cron.unschedule('nightly-vacuum' );
unschedule
------------
t
SELECT cron.unschedule(42);
unschedule
------------
t
-- Vacuum every Sunday at 4:00am (GMT) in a database other than the one pg_cron is installed in
SELECT cron.schedule_in_database('weekly-vacuum', '0 4 * * 0', 'VACUUM', 'some_other_database');
schedule
----------
44
-- Call a stored procedure every 5 seconds
SELECT cron.schedule('process-updates', '5 seconds', 'CALL process_updates()');
-- Process payroll at 12:00 of the last day of each month
SELECT cron.schedule('process-payroll', '0 12 $ * *', 'CALL process_payroll()');
```
pg_cron can run multiple jobs in parallel, but it runs at most one instance of a job at a time. If a second run is supposed to start before the first one finishes, then the second run is queued and started as soon as the first run completes.
The schedule uses the standard cron syntax, in which * means "run every time period", and a specific number means "but only at this time":
```
┌───────────── min (0 - 59)
│ ┌────────────── hour (0 - 23)
│ │ ┌─────────────── day of month (1 - 31) or last day of the month ($)
│ │ │ ┌──────────────── month (1 - 12)
│ │ │ │ ┌───────────────── day of week (0 - 6) (0 to 6 are Sunday to
│ │ │ │ │ Saturday, or use names; 7 is also Sunday)
│ │ │ │ │
│ │ │ │ │
* * * * *
```
An easy way to create a cron schedule is: [crontab.guru](http://crontab.guru/).
The code in pg_cron that handles parsing and scheduling comes directly from the cron source code by Paul Vixie, hence the same options are supported.
## Installing pg_cron
Install on Red Hat, CentOS, Fedora, Amazon Linux with PostgreSQL 16 using [PGDG](https://yum.postgresql.org/repopackages/):
```bash
# Install the pg_cron extension
sudo yum install -y pg_cron_16
```
Install on Debian, Ubuntu with PostgreSQL 16 using [apt.postgresql.org](https://wiki.postgresql.org/wiki/Apt):
```bash
# Install the pg_cron extension
sudo apt-get -y install postgresql-16-cron
```
You can also install pg_cron by building it from source:
```bash
git clone https://github.com/citusdata/pg_cron.git
cd pg_cron
# Ensure pg_config is in your path, e.g.
export PATH=/usr/pgsql-16/bin:$PATH
make && sudo PATH=$PATH make install
```
## Setting up pg_cron
To start the pg_cron background worker when PostgreSQL starts, you need to add pg_cron to `shared_preload_libraries` in postgresql.conf. Note that pg_cron does not run any jobs as a long a server is in [hot standby](https://www.postgresql.org/docs/current/static/hot-standby.html) mode, but it automatically starts when the server is promoted.
```
# add to postgresql.conf
# required to load pg_cron background worker on start-up
shared_preload_libraries = 'pg_cron'
```
By default, the pg_cron background worker expects its metadata tables to be created in the "postgres" database. However, you can configure this by setting the `cron.database_name` configuration parameter in postgresql.conf.
```
# add to postgresql.conf
# optionally, specify the database in which the pg_cron background worker should run (defaults to postgres)
cron.database_name = 'postgres'
```
`pg_cron` may only be installed to one database in a cluster. If you need to run jobs in multiple databases, use `cron.schedule_in_database()`.
Previously pg_cron could only use GMT time, but now you can adapt your time by setting `cron.timezone` in postgresql.conf.
```
# add to postgresql.conf
# optionally, specify the timezone in which the pg_cron background worker should run (defaults to GMT). E.g:
cron.timezone = 'PRC'
```
After restarting PostgreSQL, you can create the pg_cron functions and metadata tables using `CREATE EXTENSION pg_cron`.
```sql
-- run as superuser:
CREATE EXTENSION pg_cron;
-- optionally, grant usage to regular users:
GRANT USAGE ON SCHEMA cron TO marco;
```
### Ensuring pg_cron can start jobs
**Important**: By default, pg_cron uses libpq to open a new connection to the local database, which needs to be allowed by [pg_hba.conf](https://www.postgresql.org/docs/current/static/auth-pg-hba-conf.html).
It may be necessary to enable `trust` authentication for connections coming from localhost in for the user running the cron job, or you can add the password to a [.pgpass file](https://www.postgresql.org/docs/current/static/libpq-pgpass.html), which libpq will use when opening a connection.
You can also use a unix domain socket directory as the hostname and enable `trust` authentication for local connections in [pg_hba.conf](https://www.postgresql.org/docs/current/static/auth-pg-hba-conf.html), which is normally safe:
```
# Connect via a unix domain socket:
cron.host = '/tmp'
# Can also be an empty string to look for the default directory:
cron.host = ''
```
Alternatively, pg_cron can be configured to use background workers. In that case, the number of concurrent jobs is limited by the `max_worker_processes` setting, so you may need to raise that.
```
# Schedule jobs via background workers instead of localhost connections
cron.use_background_workers = on
# Increase the number of available background workers from the default of 8
max_worker_processes = 20
```
For security, jobs are executed in the database in which the `cron.schedule` function is called with the same permissions as the current user. In addition, users are only able to see their own jobs in the `cron.job` table.
```sql
-- View active jobs
select * from cron.job;
```
## Viewing job run details
You can view the status of running and recently completed job runs in the `cron.job_run_details`:
```sql
select * from cron.job_run_details order by start_time desc limit 5;
┌───────┬───────┬─────────┬──────────┬──────────┬───────────────────┬───────────┬──────────────────┬───────────────────────────────┬───────────────────────────────┐
│ jobid │ runid │ job_pid │ database │ username │ command │ status │ return_message │ start_time │ end_time │
├───────┼───────┼─────────┼──────────┼──────────┼───────────────────┼───────────┼──────────────────┼───────────────────────────────┼───────────────────────────────┤
│ 10 │ 4328 │ 2610 │ postgres │ marco │ select process() │ succeeded │ SELECT 1 │ 2023-02-07 09:30:00.098164+01 │ 2023-02-07 09:30:00.130729+01 │
│ 10 │ 4327 │ 2609 │ postgres │ marco │ select process() │ succeeded │ SELECT 1 │ 2023-02-07 09:29:00.015168+01 │ 2023-02-07 09:29:00.832308+01 │
│ 10 │ 4321 │ 2603 │ postgres │ marco │ select process() │ succeeded │ SELECT 1 │ 2023-02-07 09:28:00.011965+01 │ 2023-02-07 09:28:01.420901+01 │
│ 10 │ 4320 │ 2602 │ postgres │ marco │ select process() │ failed │ server restarted │ 2023-02-07 09:27:00.011833+01 │ 2023-02-07 09:27:00.72121+01 │
│ 9 │ 4320 │ 2602 │ postgres │ marco │ select do_stuff() │ failed │ job canceled │ 2023-02-07 09:26:00.011833+01 │ 2023-02-07 09:26:00.22121+01 │
└───────┴───────┴─────────┴──────────┴──────────┴───────────────────┴───────────┴──────────────────┴───────────────────────────────┴───────────────────────────────┘
(10 rows)
```
The records in `cron.job_run_details` are not cleaned automatically, but every user that can schedule cron jobs also has permission to delete their own `cron.job_run_details` records.
Especially when you have jobs that run every few seconds, it can be a good idea to clean up regularly, which can easily be done using pg_cron itself:
```sql
-- Delete old cron.job_run_details records of the current user every day at noon
SELECT cron.schedule('delete-job-run-details', '0 12 * * *', $$DELETE FROM cron.job_run_details WHERE end_time < now() - interval '7 days'$$);
```
If you do not want to use `cron.job_run_details` at all, then you can add `cron.log_run = off` to `postgresql.conf`.
## Example use cases
Articles showing possible ways of using pg_cron:
* [Auto-partitioning using pg_partman](https://www.citusdata.com/blog/2018/01/24/citus-and-pg-partman-creating-a-scalable-time-series-database-on-postgresql/)
* [Computing rollups in an analytical dashboard](https://www.citusdata.com/blog/2017/12/27/real-time-analytics-dashboards-with-citus/)
* [Deleting old data, vacuum](https://www.citusdata.com/blog/2016/09/09/pgcron-run-periodic-jobs-in-postgres/)
* [Feeding cats](http://bonesmoses.org/2016/09/09/pg-phriday-irrelevant-inclinations/)
* [Routinely invoking a function](https://fluca1978.github.io/2019/05/21/pgcron.html)
* [Postgres as a cron server](https://supabase.io/blog/2021/03/05/postgres-as-a-cron-server)
## Managed services
The following table keeps track of which of the major managed Postgres services support pg_cron.
| Service | Supported |
| ------------- |:-------------:|
| [Aiven](https://aiven.io/postgresql) | :heavy_check_mark: |
| [Alibaba Cloud](https://www.alibabacloud.com/help/doc-detail/150355.htm) | :heavy_check_mark: |
| [Amazon RDS](https://aws.amazon.com/rds/postgresql/) | :heavy_check_mark: | |
| [Azure](https://azure.microsoft.com/en-us/services/postgresql/) | :heavy_check_mark: |
| [Crunchy Bridge](https://www.crunchydata.com/products/crunchy-bridge/?ref=producthunt) | :heavy_check_mark: |
| [DigitalOcean](https://www.digitalocean.com/products/managed-databases/) | :heavy_check_mark: |
| [Google Cloud](https://cloud.google.com/sql/postgresql/) | :heavy_check_mark: |
| [Heroku](https://elements.heroku.com/addons/heroku-postgresql) | :x: |
| [Instaclustr](https://instaclustr.com) | :heavy_check_mark: |
| [ScaleGrid](https://scalegrid.io/postgresql.html) | :heavy_check_mark: |
| [Scaleway](https://www.scaleway.com/en/database/) | :heavy_check_mark: |
| [Supabase](https://supabase.io/docs/guides/database) | :heavy_check_mark: |
| [Tembo](https://tembo.io) | :heavy_check_mark: |
| [YugabyteDB](https://www.yugabyte.com/) | :heavy_check_mark: |
## Code of Conduct
This project has adopted the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/). For more information see the [Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or contact [
[email protected]](mailto:
[email protected]) with any additional questions or comments.
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