Scheduling maintenance with the PostgreSQL pg_cron extension - Amazon Relational Database Service

Scheduling maintenance with the PostgreSQL pg_cron extension

The PostgreSQL pg_cron extension provides a scheduler for maintenance commands within a PostgreSQL database. See also What is pg_cron? in the pg_cron documentation.

Note

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

Enabling the pg_cron extension

Enable the pg_cron extension as follows:

  1. Modify the parameter group associated with your DB instance and add pg_cron to the shared_preload_libraries parameter value. This change requires a DB instance restart for it to take effect. For more information, see Modifying parameters in a DB parameter group.

  2. After the DB instance has restarted, perform the following command using an account that has the rds_superuser permissions:

    CREATE EXTENSION pg_cron;
  3. The pg_cron scheduler is set in the default PostgreSQL database named postgres. The pg_cron objects are created in this postgres database and all scheduling actions run in this database.

    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 postgres.

Granting permissions to pg_cron

As the rds_superuser, you can create the pg_cron extension and then grant permissions to other users. For other users to be able to schedule jobs, grant them permissions to objects in the cron schema.

Important

We recommend that you grant permissions to the cron schema sparingly.

To grant others permission to the cron schema, do the following:

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

This permission provides the other-user with access to the cron schema to schedule and unschedule cron jobs. However, for the cron jobs to execute successfully, the user also needs permission to access the objects in the cron jobs. If the user does not have permission, the job will fail and errors such as the following will appear in the postgresql.log. In this example, the user does not have permission to access the pgbench_accounts table.

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

There will also be messages in the cron.job_run_details table such as the following:

postgres=> select jobid, username, status, return_message, start_time from cron.job_run_details where status = 'failed'; jobid | username | status | return_message | start_time -------+------------+--------+-----------------------------------------------------+------------------------------- 143 | unprivuser | failed | ERROR: permission denied for table pgbench_accounts | 2020-12-08 16:41:00.036268+00 143 | unprivuser | failed | ERROR: permission denied for table pgbench_accounts | 2020-12-08 16:40:00.050844+00 143 | unprivuser | failed | ERROR: permission denied for table pgbench_accounts | 2020-12-08 16:42:00.175644+00 143 | unprivuser | failed | ERROR: permission denied for table pgbench_accounts | 2020-12-08 16:43:00.069174+00 143 | unprivuser | failed | ERROR: permission denied for table pgbench_accounts | 2020-12-08 16:44:00.059466+00 (5 rows)

See also The pg_cron tables.

Cron job to manually vacuum a table

Autovacuum handles vacuum maintenance for most cases. See Working with PostgreSQL autovacuum on Amazon RDS. However, you might want to manually vacuum a specific table at a time of your choosing. 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 this 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 an example of viewing the history in the cron.job_run_details table to investigate the cause of a job that failed.

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)

See also The pg_cron tables.

Cron job to purge the pg_cron history

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 may 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 * * *', $$DELETE FROM cron.job_run_details WHERE end_time < now() – interval '7 days'$$);

See also The pg_cron tables.

Disabling logging of pg_cron history

To completely disable writing anything to the cron.job_run_details table, modify the parameter group associated with the DB instance and set the cron.log_run parameter to off. The pg_cron extension will no longer write to the table and will only produce errors in the postgresql.log file. See also Modifying parameters in a DB parameter group. Use the following command to check the value of the cron.log_run parameter.

postgres=> SHOW cron.log_run;

See also The pg_cron parameters.

Scheduling a cron job for a database other than postgres

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

  1. In the cron database, schedule the job as you normally would using the cron.schedule function:

    postgres=> SELECT cron.schedule('database1 manual vacuum', '29 03 * * *', 'vacuum freeze test_table');
  2. As an rds_superuser, update the database column for the job you just created so that it runs in another database within your RDS DB instance.

    postgres=> UPDATE cron.job SET database = 'database1' WHERE jobid = 106;
  3. Verify by querying the cron.job table.

    postgres=> select * from cron.job; jobid | schedule | command | nodename | nodeport | database | username | active | jobname -------+-------------+----------------------------------------+-----------+----------+-----------+-----------+--------+------------------------- 106 | 29 03 * * * | vacuum freeze test_table | localhost | 8192 | database1 | adminuser | t | database1 manual vacuum 1 | 59 23 * * * | vacuum freeze pgbench_accounts | localhost | 8192 | postgres | adminuser | t | manual vacuum (2 rows)
Note

When you add a cron job that you intend to have execute on a different database, it is possible for the job to attempt to run in the default database (postgres) before you update the correct database column. If the user name has permissions, it would successfully execute in the default database.

The pg_cron reference

The following parameters, functions, and tables are available with the pg_cron extension. For more information, see What is pg_cron? in the pg_cron documentation.

The pg_cron parameters

Following is the list of parameters to control the pg_cron extension behavior.

Parameter Description

cron.database_name

The database in which pg_cron metadata is kept.

cron.host

The hostname to connect to PostgresSQL. You can't modify this value.

cron.log_run

Log into the job_run_details table all jobs that run . Values are on or off.

See also The pg_cron tables.

cron.log_statement

Log all cron statements prior to execution. Values are on or off.

cron.max_running_jobs

The maximum number of jobs that can run concurrently.

cron.use_background_workers

Use background workers instead of client sessions. You can't modify this value.

You can 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;

cron.schedule function

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 postgres.

The function has two syntax formats.

Syntax
cron.schedule (job_name, schedule, command ); cron.schedule (schedule, command );
Parameters
Parameter Description
job_name

The name of the cron job.

schedule

Text indicating the schedule for the cron job. The format is the standard cron format.

command Text of the command to execute.
Examples
postgres=> SELECT cron.schedule ('test','0 10 * * *', 'VACUUM pgbench_history'); schedule ---------- 145 (1 row) postgres=> SELECT cron.schedule ('0 15 * * *', 'VACUUM pgbench_accounts'); schedule ---------- 146 (1 row)

cron.unschedule function

This function deletes a cron job. You can either pass in the job_name or the job_id. A policy makes sure you are the owner in order to unschedule the job. The function returns a Boolean indicating success or failure.

The function has the following syntax formats:

Syntax
cron.unschedule (job_id); cron.unschedule (job_name);
Parameters
Parameter Description
job_id

A job identifier that was returned from the cron.schedule function when the cron job was scheduled.

job_name

The name of a cron job that was scheduled with the cron.schedule function.

Examples
postgres=> select cron.unschedule(108); unschedule ------------ t (1 row) postgres=> select cron.unschedule('test'); unschedule ------------ t (1 row)

The pg_cron tables

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

Table Name Description
cron.job

Contains the metadata about each scheduled job. Most interactions with this table should be done via the cron.schedule and cron.unschedule functions.

Note

We do not recommend giving update/insert privileges directly to this table. Doing so would allow the user to update the username column to run as rds-superuser.

cron.job_run_details

Contains historic information about past scheduled job executions. This is useful to investigate the status, return messages, and start and end time from the job execution.

Note

This table should be purged on a regular basis so that it doesn’t grow indefinitely. For an example, see Cron job to purge the pg_cron history.