Known limitations

Overview

Before you choose to migrate your databases to Cloud SQL, make sure you consider known limitations for this migration scenario.

Known limitations for using a PostgreSQL database as a source include:

• The pglogical extension doesn't support the replication of generated columns for PostgreSQL 12+.

• Changes to table structures (DDL) aren't replicated through standard DDL commands, but only with commands executed using the pglogical extension used for replication. This includes changes to enum types.

  • For example, pglogical provides a function pglogical.replicate_ddl_command that allows DDL to be run on both the source database and replica at a consistent point. The user running this command on the source must already exist on the replica.

  • In order to replicate data for new tables, you must use the pglogical.replication_set_add_table command to add the new tables to existing replication sets.

  • To learn more about DDL replication while the migration is in progress, see the section on migration fidelity.

• For tables that don't have primary keys, Database Migration Service supports migration of the initial snapshot and INSERT statements during the change data capture (CDC) phase. You should migrate UPDATE and DELETE statements manually.

• Database Migration Service doesn't migrate data from materialized views, just the view schema. To populate the views, run the following command: REFRESH MATERIALIZED VIEW view_name.

• The SEQUENCE states (for example, last_value) on the new Cloud SQL destination might vary from the source SEQUENCE states.

• UNLOGGED and TEMPORARY tables aren't and can't be replicated.

• Large Object data type isn't supported. More details in the section on migration fidelity.

• Only extensions and procedural languages that Cloud SQL supports for PostgreSQL can be migrated. Database Migration Service doesn't migrate extensions that are unsupported by Cloud SQL. The presence of these extensions doesn't block the migration, but to ensure a smooth migration process verify that your objects or applications don't reference any unsupported extensions. We recommend removing these extensions and references from your source database before you proceed.

• The pg_cron extension (or any cron settings associated with the extension) isn't migrated by Database Migration Service, but it is supported in Cloud SQL for PostgreSQL destinations. If you use the pg_cron extension in your source databases, you can re-install it on your destination instance after the migration is complete.

• Database Migration Service doesn't support migrating from read replicas that are in recovery mode.

• Database Migration Service doesn't support Amazon RDS sources where the AWS SCT extension pack is applied.

• User-defined functions written in C can't be migrated, except for functions that are installed in the PostgreSQL database when you're installing extensions that are supported by Cloud SQL.

• If other extensions and procedural languages exist in the source database, or if their versions aren't supported, then when you test or start the migration job, it will fail.

• Databases that are added after the migration job has started aren't migrated.

• You can't select specific tables or schemas when you migrate by using Database Migration Service. Database Migration Service migrates all tables and schemas, except for the following:
  • The information schema (information_schema).
  • Any tables that begin with pg, for example, pg_catalog. For the full list of PostgreSQL catalogs that begin with pg, see PostgreSQL system catalogs in the PostgreSQL documentation.
  • Information about users and user roles isn't migrated.

• If encrypted databases require customer-managed encryption keys to decrypt the databases, and if Database Migration Service doesn't have access to the keys, then the databases can't be migrated.

  However, if customer data is encrypted by the pgcrypto extension, then the data can be migrated with Database Migration Service (because Cloud SQL supports the extension).

  Database Migration Service also supports migrating data from encrypted Amazon Aurora or Amazon RDS databases because these databases handle decryption transparently in their services. For more information, see Encrypting Amazon Aurora resources and Encrypting Amazon RDS resources.

• The destination Cloud SQL database is writable during the migration to allow DDL changes to be applied if needed. Take care not to make any changes to the database configuration or table structures which might break the migration process or impact data integrity.

• Trigger behavior depends on how they were configured. The default behavior is they won't trigger, but if they were configured using the ALTER EVENT TRIGGER or ALTER TABLE statement and the trigger state is set to either replica or always, then they will trigger on the replica during replication.

• Functions with security definer will be created by cloudsqlexternalsync in Cloud SQL replica. When it's executed by any users, it will be executed with the privileges of cloudsqlexternalsync which has cloudsqlsuperuser and cloudsqlreplica roles. It's better to restrict use of a security definer function to only some users. To do that, the user should revoke the default PUBLIC privileges and then grant execute privilege selectively.

• Cloud SQL does not support customized tablespaces. All the data inside customized tablespaces is migrated to the pg_default tablespace in the Cloud SQL destination instance.

• Private Service Connect interfaces connectivity method is only supported for migrating to existing destination instances. If you want to use private IP connectivity and migrate to a new destination instance, use VPC peering.

  Limitations for migrations to existing destination instances

  • Your existing destination instance must be empty or contain only system configuration data. Migrating to existing destination instances that contain user data (such as tables) isn't supported.

    If you encounter issues due to extra data in your existing destination instance, clear the databases in your destination instance and re-try the migration job. See Clear extra data from your existing destination instance.

  • You can configure only one migration job per destination instance.
  • You can only migrate to standalone Cloud SQL instances. Migrating to external server replicas isn't supported.
  • Migrating data to a Cloud SQL instance that has Private Service Connect enabled isn't supported.
  • After you promote an instance, you must turn on point-in-time recovery.
  • If your instance has customized backup settings (for example, a custom backup location), then after you promote the instance, you must customize your backup settings again. During the promotion process, Cloud SQL resets your backup settings to their default values.
  • For Terraform users: Database Migration Service modifies the backup and recovery settings of your destination instance. This might cause the destination instance settings to be different from the Terraform configuration you used for provisioning. If you experience this issue, follow the guidance in Diagnose issues.

  Quotas

  • Up to 2,000 connection profiles and 1,000 migration jobs can exist at any given time. To create space for more, migration jobs (including completed ones) and connection profiles can be deleted.