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doc: Add section about logical replication restrictions

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Peter Eisentraut
2017-06-16 09:55:55 -04:00
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doc/src/sgml/logical-replication.sgml
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@ -299,6 +299,76 @@
</para> </para>
</sect1> </sect1>
<sect1 id="logical-replication-restrictions">
<title>Restrictions</title>
<para>
Logical replication currently has the following restrictions or missing
functionality. These might be addressed in future releases.
</para>
<itemizedlist>
<listitem>
<para>
The database schema and DDL commands are not replicated. The initial
schema can be copied by hand using <literal>pg_dump
--schema-only</literal>. 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.
</para>
</listitem>
<listitem>
<para>
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 <command>pg_dump</command>) or by determining a sufficiently high
value from the tables themselves.
</para>
</listitem>
<listitem>
<para>
<command>TRUNCATE</command> commands are not replicated. This can, of
course, be worked around by using <command>DELETE</command> instead. To
avoid accidental <command>TRUNCATE</command> invocations, you can revoke
the <literal>TRUNCATE</literal> privilege from tables.
</para>
</listitem>
<listitem>
<para>
Large objects (see <xref linkend="largeobjects">) are not replicated.
There is no workaround for that, other than storing data in normal
tables.
</para>
</listitem>
<listitem>
<para>
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.
</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="logical-replication-architecture"> <sect1 id="logical-replication-architecture">
<title>Architecture</title> <title>Architecture</title>