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Scheduling maintenance with the PostgreSQL pg_cron extension

You can use the PostgreSQL pg_cron extension to schedule maintenance commands within a PostgreSQL database. For more information about the extension, see What is pg_cron? in the pg_cron documentation.

The pg_cron extension is supported on RDS for PostgreSQL engine versions 12.5 and higher.

To learn more about using pg_cron, see Schedule jobs with pg_cron on your RDS for PostgreSQL or your Aurora PostgreSQL-Compatible Edition databases.

Setting up the pg_cron extension

Set up the pg_cron extension as follows:

Granting database users permissions to use pg_cron

Installing the pg_cron extension requires the rds_superuser privileges. However, permissions to use the pg_cron can be granted (by a member of the rds_superuser group/role) to other database users, so that they can schedule their own jobs. We recommend that you grant permissions to the cron schema only as needed if it improves operations in your production environment.

To grant a database user permission in the cron schema, run the following command:

postgres=> GRANT USAGE ON SCHEMA cron TO db-user;

This gives db-user permission to access the cron schema to schedule cron jobs for the objects that they have permissions to access. If the database user doesn't have permissions, the job fails after posting the error message to the postgresql.log file, as shown in the following:

2020-12-08 16:41:00 UTC::@:[30647]:ERROR: permission denied for table table-name
2020-12-08 16:41:00 UTC::@:[27071]:LOG: background worker "pg_cron" (PID 30647) exited with exit code 1

In other words, make sure that database users that are granted permissions on the cron schema also have permissions on the objects (tables, schemas, and so on) that they plan to schedule.

The details of the cron job and its success or failure are also captured in the cron.job_run_details table. For more information, see Tables for scheduling jobs and capturing status .

Scheduling pg_cron jobs

The following sections show how you can schedule various management tasks using pg_cron jobs.

Vacuuming a table

Autovacuum handles vacuum maintenance for most cases. However, you might want to schedule a vacuum of a specific table at a time of your choosing.

See also, Working with PostgreSQL autovacuum on Amazon RDS for PostgreSQL.

Following is an example of using the cron.schedule function to set up a job to use VACUUM FREEZE on a specific table every day at 22:00 (GMT).

SELECT cron.schedule('manual vacuum', '0 22 * * *', 'VACUUM FREEZE pgbench_accounts');
 schedule
----------
1
(1 row)

After the preceding example runs, you can check the history in the cron.job_run_details table as follows.

postgres=> SELECT * FROM cron.job_run_details;
jobid  | runid | job_pid | database | username | command                        | status    | return_message | start_time                    | end_time
-------+-------+---------+----------+----------+--------------------------------+-----------+----------------+-------------------------------+-------------------------------
 1     | 1     | 3395    | postgres | adminuser| vacuum freeze pgbench_accounts | succeeded | VACUUM         | 2020-12-04 21:10:00.050386+00 | 2020-12-04 21:10:00.072028+00
(1 row)

Following is a query of the cron.job_run_details table to see the failed jobs.

postgres=> SELECT * FROM cron.job_run_details WHERE status = 'failed';
jobid | runid | job_pid | database | username | command                       | status | return_message                                   | start_time                    | end_time
------+-------+---------+----------+----------+-------------------------------+--------+--------------------------------------------------+-------------------------------+------------------------------
 5    | 4     | 30339   | postgres | adminuser| vacuum freeze pgbench_account | failed | ERROR: relation "pgbench_account" does not exist | 2020-12-04 21:48:00.015145+00 | 2020-12-04 21:48:00.029567+00
(1 row)

For more information, see Tables for scheduling jobs and capturing status .

Purging the pg_cron history table

The cron.job_run_details table contains a history of cron jobs that can become very large over time. We recommend that you schedule a job that purges this table. For example, keeping a week's worth of entries might be sufficient for troubleshooting purposes.

The following example uses the cron.schedule function to schedule a job that runs every day at midnight to purge the cron.job_run_details table. The job keeps only the last seven days. Use your rds_superuser account to schedule the job such as the following.

SELECT cron.schedule('0 0 * * *', $$DELETE 
    FROM cron.job_run_details 
    WHERE end_time < now() - interval '7 days'$$);

For more information, see Tables for scheduling jobs and capturing status .

Logging errors to the postgresql.log file only

To prevent writing to the cron.job_run_details table, modify the parameter group associated with the PostgreSQL DB instance and set the cron.log_run parameter to off. The pg_cron extension no longer writes to the table and captures errors to the postgresql.log file only. For more information, see Modifying parameters in a DB parameter group in Amazon RDS.

Use the following command to check the value of the cron.log_run parameter.

postgres=> SHOW cron.log_run;

For more information, see Parameters for managing the pg_cron extension.

Scheduling a cron job for a database other than the default database

The metadata for pg_cron is all held in the PostgreSQL default database named postgres. Because background workers are used for running the maintenance cron jobs, you can schedule a job in any of your databases within the PostgreSQL DB instance:

Reference for the pg_cron extension

You can use the following parameters, functions, and tables with the pg_cron extension. For more information, see What is pg_cron? in the pg_cron documentation.

Parameters for managing the pg_cron extension

Following is a list of parameters that control the pg_cron extension behavior.

Use the following SQL command to display these parameters and their values.

postgres=> SELECT name, setting, short_desc FROM pg_settings WHERE name LIKE 'cron.%' ORDER BY name;

Function reference: cron.schedule

This function schedules a cron job. The job is initially scheduled in the default postgres database. The function returns a bigint value representing the job identifier. To schedule jobs to run in other databases within your PostgreSQL DB instance, see the example in Scheduling a cron job for a database other than the default database.

The function has two syntax formats.

Function reference: cron.unschedule

This function deletes a cron job. You can specify either the job_name or the job_id. A policy makes sure that you are the owner to remove the schedule for the job. The function returns a Boolean indicating success or failure.

The function has the following syntax formats.

Tables for scheduling jobs and capturing status

The following tables are used to schedule the cron jobs and record how the jobs completed.