30.4. Restrictions

The database schema and DDL commands are not replicated. The initial schema can be copied by hand using pg_dump --schema-only . Subsequent schema changes would need to be kept in sync manually. (Note, however, that there is no need for the schemas to be absolutely the same on both sides.) Logical replication is robust when schema definitions change in a live database: When the schema is changed on the publisher and replicated data starts arriving at the subscriber but does not fit into the table schema, replication will error until the schema is updated. In many cases, intermittent errors can be avoided by applying additive schema changes to the subscriber first.

Sequence data is not replicated. The data in serial or identity columns backed by sequences will of course be replicated as part of the table, but the sequence itself would still show the start value on the subscriber. If the subscriber is used as a read-only database, then this should typically not be a problem. If, however, some kind of switchover or failover to the subscriber database is intended, then the sequences would need to be updated to the latest values, either by copying the current data from the publisher (perhaps using pg_dump ) or by determining a sufficiently high value from the tables themselves.

Replication of TRUNCATE commands is supported, but some care must be taken when truncating groups of tables connected by foreign keys. When replicating a truncate action, the subscriber will truncate the same group of tables that was truncated on the publisher, either explicitly specified or implicitly collected via CASCADE , minus tables that are not part of the subscription. This will work correctly if all affected tables are part of the same subscription. But if some tables to be truncated on the subscriber have foreign-key links to tables that are not part of the same (or any) subscription, then the application of the truncate action on the subscriber will fail.

Large objects (see Chapter 34 ) are not replicated. There is no workaround for that, other than storing data in normal tables.

Replication is only possible from base tables to base tables. That is, the tables on the publication and on the subscription side must be normal tables, not views, materialized views, partition root tables, or foreign tables. In the case of partitions, you can therefore replicate a partition hierarchy one-to-one, but you cannot currently replicate to a differently partitioned setup. Attempts to replicate tables other than base tables will result in an error.