Partitioned tables can now be replicated

…a bit more conveniently

May 14, 2020

If your shop uses logical replication to selectively replicate tables between two PostgreSQL clusters and you also happen to rely on partitioning for some of these tables, you may have gotten annoyed by seeing this error on the publishing server:

ERROR:  "foo" is a partitioned table
DETAIL:  Adding partitioned tables to publications is not supported.
HINT:  You can add the table partitions individually.

That is, you cannot replicate partitioned tables directly. As the hint says, you can replicate the individual (leaf) partitions by explicitly adding them to the publication, provided the set of partitions on the receiving server matches one-to-one with the published partitions. It is perhaps manageable by having your partitioning DDL script also have some code to manage the publication status of individual partitions.

With Postgres 13, partitioned tables can now be directly added to publications. By default, publishing a partitioned table by adding it to publication is really just a shorthand for publishing all of its present and future leaf partitions. This still implies though that matching partitions must be present on the receiving server. Actually, INSERT, UPDATE, DELETE operations on a partitioned table are physically applied to its leaf partitions and so each operation’s WAL record (or specifically, the logical information contained in it) contains the leaf partitions’ schema information. Because logical replication publisher plugin (pgoutput) generates logical change records to publish by decoding WAL, those records thus contain leaf partition information. That is why the receiving server must have the same leaf partitions present to be able to consume the changes.

If that sounds too restrictive, you may want to try the new publication parameter publish_via_partition_root, whereby the changes of partitioned tables contained in a publication are published using their own schema, instead of that of the leaf partitions that are actually changed. Example:

CREATE PUBLICATION "pub" FOR TABLE "foo" WITH (publish_via_partition_root = true);

With this, for each decoded change, pgoutput maps the leaf partition information contained in decoded change record back to the ancestor that is actually present in the publication. That ancestor typically would be the root partitioned table, in which case, changes of all leaf partitions contained in the partition tree are published using the root partitioned table’s schema. One may also publish only a subtree of a multi-level partition tree by adding the subtree’s root table to the publication, in which case only leaf partitions of that subtree are published.

The above feature to replicate using an ancestor’s schema allows the receiving server to consume the received changes without having the matching leaf partitions present, as is required by default. The changes can be received into either a non-partitioned table or a partitioned table of the same name as the published ancestor. The receiving partitioned table may also contain a different set of partitions than those of the published partitioned table.

In fact, receiving changes into a partitioned table is also something that wasn’t allowed before; you may have seen the following error on a receiving server:

create subscription sub connection '$connection_string' publication pub;
ERROR:  cannot use relation "" as logical replication target
DETAIL:  "" is a partitioned table.

Upgrading the receiving server to Postgres 13 will fix that for you. The receiving server now accepts the changes to be received into partitioned tables. Those changes are routed to the correct partitions, which of course is based on the locally defined partition key and bounds and has nothing to do with which partition the affected rows were present in on the publishing server.

To summarize:

  1. On publishing server, you can now add partitioned tables directly to publications, causing the changes of its leaf partitions to be published.

  2. Changes can now be published using the schema of the ancestor (typically the root partitioned table) that’s actually present in the publication by enabling the new publication parameter publish_via_partition_root. It is disabled by default, meaning the changes are published using the name of the leaf partition that is actually changed by the replicated operation.

  3. On receiving server, changes can now be received into partitioned tables, whereas previously only non-partitioned tables were allowed, which allows to replicate between paritioned tables with different sets of partitions.

If performance of replication is something you care about, then it’s better to avoid relying on #2 and #3 above, because there is a certain overhead associated with it, even though very minimal. Especially, if you keep a matching set of leaf partitions on the two servers anyway, using only #1 might be enough for you.

© 2023 Amit Langote. Hosted by GitHub. Powered by Jekyll