Managing large objects with the lo module - Amazon Aurora

Managing large objects with the lo module

The lo module (extension) is for database users and developers working with PostgreSQL databases through JDBC or ODBC drivers. Both JDBC and ODBC expect the database to handle deletion of large objects when references to them change. However, PostgreSQL doesn't work that way. PostgreSQL doesn't assume that an object should be deleted when its reference changes. The result is that objects remain on disk, unreferenced. The lo extension includes a function that you use to trigger on reference changes to delete objects if needed.

Tip

To determine if your database can benefit from the lo extension, use the vacuumlo utility to check for orphaned large objects. To get counts of orphaned large objects without taking any action, run the utility with the -n option (no-op). To learn how, see vacuumlo utility following.

The lo module is available for Aurora PostgreSQL 13.7, 12.11, 11.16, 10.21 and higher minor versions.

To install the module (extension), you need rds_superuser privileges. Installing the lo extension adds the following to your database:

  • lo – This is a large object (lo) data type that you can use for binary large objects (BLOBs) and other large objects. The lo data type is a domain of the oid data type. In other words, it's an object identifier with optional constraints. For more information, see Object identifiers in the PostgreSQL documentation. In simple terms, you can use the lo data type to distinguish your database columns that hold large object references from other object identifiers (OIDs).

  • lo_manage – This is a function that you can use in triggers on table columns that contain large object references. Whenever you delete or modify a value that references a large object, the trigger unlinks the object (lo_unlink) from its reference. Use the trigger on a column only if the column is the sole database reference to the large object.

For more information about the large objects module, see lo in the PostgreSQL documentation.

Installing the lo extension

Before installing the lo extension, make sure that you have rds_superuser privileges.

To install the lo extension

  1. Use psql to connect to the primary DB instance of your Aurora PostgreSQL DB cluster.

    psql --host=your-cluster-instance-1.666666666666.aws-region.rds.amazonaws.com --port=5432 --username=postgres --password

    When prompted, enter your password. The psql client connects and displays the default administrative connection database, postgres=>, as the prompt.

  2. Install the extension as follows.

    postgres=> CREATE EXTENSION lo; CREATE EXTENSION

You can now use the lo data type to define columns in your tables. For example, you can create a table (images) that contains raster image data. You can use the lo data type for a column raster, as shown in the following example, which creates a table.

postgres=> CREATE TABLE images (image_name text, raster lo);

Using the lo_manage trigger function to delete objects

You can use the lo_manage function in a trigger on a lo or other large object columns to clean up (and prevent orphaned objects) when the lo is updated or deleted.

To set up triggers on columns that reference large objects

  • Do one of the following:

    • Create a BEFORE UPDATE OR DELETE trigger on each column to contain unique references to large objects, using the column name for the argument.

      postgres=> CREATE TRIGGER t_raster BEFORE UPDATE OR DELETE ON images FOR EACH ROW EXECUTE FUNCTION lo_manage(raster);
    • Apply a trigger only when the column is being updated.

      postgres=> CREATE TRIGGER t_raster BEFORE UPDATE OF images FOR EACH ROW EXECUTE FUNCTION lo_manage(raster);

The lo_manage trigger function works only in the context of inserting or deleting column data, depending on how you define the trigger. It has no effect when you perform a DROP or TRUNCATE operation on a database. That means that you should delete object columns from any tables before dropping, to prevent creating orphaned objects.

For example, suppose that you want to drop the database containing the images table. You delete the column as follows.

postgres=> DELETE FROM images COLUMN raster

Assuming that the lo_manage function is defined on that column to handle deletes, you can now safely drop the table.

Using the vacuumlo utility

The vacuumlo utility identifies and can remove orphaned large objects from databases. This utility has been available since PostgreSQL 9.1.24. If your database users routinely work with large objects, we recommend that you run vacuumlo occasionally to clean up orphaned large objects.

Before installing the lo extension, you can use vacuumlo to assess whether your Aurora PostgreSQL DB cluster can benefit. To do so, run vacuumlo with the -n option (no-op) to show what would be removed, as shown in the following:

$ vacuumlo -v -n -h your-cluster-instance-1.666666666666.aws-region.rds.amazonaws.com -p 5433 -U postgres docs-lab-spatial-db Password:***** Connected to database "docs-lab-spatial-db" Test run: no large objects will be removed! Would remove 0 large objects from database "docs-lab-spatial-db".

As the output shows, orphaned large objects aren't a problem for this particular database.

For more information about this utility, see vacuumlo in the PostgreSQL documentation.