Asynchronous Tasks

Motivation

Index creation on a large lable is a time-consuming operation. Vanilla Postgres does not have a built-in mechanism to run such time-consuming operations in the background, so a client must maintain a connection to the database for the whole duration of such an operation. In some cases, it is not desirable to keep a connection open for a long time for these operations. For example, when the backend interacting with the database is in a serverless environment.

For such cases, Lantern provides an API to run arbitrary queries in the background.

Start an asynchronous query

sql

SELECT lantern.async_task('QUERY_STRING');

For example, to launch an index creation in the background on column v of table my_table:

sql

SELECT lantern.async_task('CREATE INDEX ON my_table USING lantern_hnsw(v) WITH (m=16, ef_construction=8, ef=16);');

The lantern.async_task function returns a jobid which can be used to track the progress of the task.

The lantern.async_task also accepts an optional job_name parameter which can be used to give a name to the task.

sql

SELECT lantern.async_task('CREATE INDEX ON my_table USING lantern_hnsw(v) WITH (m=16, ef_construction=8, ef=16);', 'Create index on my_table');

You can view progress of asynchronous tasks by querying the lantern.tasks table which has the structure below:

bash

Table "lantern.tasks"
      Column      |           Type           | Collation | Nullable |                   Default
------------------+--------------------------+-----------+----------+----------------------------------------------
 jobid            | bigint                   |           | not null | nextval('lantern.tasks_jobid_seq'::regclass)
 query            | text                     |           | not null |
 pg_cron_job_name | text                     |           |          |
 job_name         | text                     |           |          |
 username         | text                     |           | not null | CURRENT_USER
 started_at       | timestamp with time zone |           | not null | now()
 duration         | interval                 |           |          |
 status           | text                     |           |          |
 error_message    | text                     |           |          |

Setup

The easiest way to use asynchronous tasks is with Lantern Cloud, which supports asynchronous tasks out-of-the-box on the paid plan. Currently, asynchronous tasks are not supported on the free plan.

To use asynchronous tasks in a self-hosted environment, ensure that both the pg_cron and lantern extensions are installed. Lantern Suite is a Docker image that comes with both extensions pre-installed.

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