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Doc: be clearer that foreign-table partitions need user-added constraints. 

A very well-informed user might deduce this from what we said already,
but I'd bet against it.  Lay it out explicitly.

While here, rewrite the comment about tuple routing to be more
intelligible to an average SQL user.

Per bug #17395 from Alexander Lakhin.  Back-patch to v11.  (The text
in this area is different in v10 and I'm not sufficiently excited
about this point to adapt the patch.)

Discussion: https://postgr.es/m/17395-8c326292078d1a57@postgresql.org
This commit is contained in:
Tom Lane 2022-02-05 12:55:44 -05:00
parent 9c7cf083f2
commit 7d74ff8243
2 changed files with 29 additions and 10 deletions
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9
doc/src/sgml/ddl.sgml
View File

@ -3719,9 +3719,12 @@ VALUES ('Albany', NULL, NULL, 'NY');
</para>
<para>
Partitions can also be foreign tables, although they have some limitations
that normal tables do not; see <xref linkend="sql-createforeigntable"/> for
more information.
Partitions can also be <link linkend="ddl-foreign-data">foreign
tables</link>, although considerable care is needed because it is then
the user's responsibility that the contents of the foreign table
satisfy the partitioning rule. There are some other restrictions as
well. See <xref linkend="sql-createforeigntable"/> for more
information.
</para>
<sect3 id="ddl-partitioning-declarative-example">

30
doc/src/sgml/ref/create_foreign_table.sgml
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@ -348,11 +348,33 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
constraints on foreign tables, it does assume that they are correct
for purposes of query optimization. If there are rows visible in the
foreign table that do not satisfy a declared constraint, queries on
the table might produce incorrect answers. It is the user's
the table might produce errors or incorrect answers. It is the user's
responsibility to ensure that the constraint definition matches
reality.
</para>
<caution>
<para>
When a foreign table is used as a partition of a partitioned table,
there is an implicit constraint that its contents must satisfy the
partitioning rule. Again, it is the user's responsibility to ensure
that that is true, which is best done by installing a matching
constraint on the remote server.
</para>
</caution>
<para>
Within a partitioned table containing foreign-table partitions,
an <command>UPDATE</command> that changes the partition key value can
cause a row to be moved from a local partition to a foreign-table
partition, provided the foreign data wrapper supports tuple routing.
However it is not currently possible to move a row from a
foreign-table partition to another partition.
An <command>UPDATE</command> that would require doing that will fail
due to the partitioning constraint, assuming that that is properly
enforced by the remote server.
</para>
<para>
Similar considerations apply to generated columns. Stored generated
columns are computed on insert or update on the local
@ -362,12 +384,6 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
generated columns that are consistent with the generation expression.
Again, this might result in incorrect query results.
</para>
<para>
While rows can be moved from local partitions to a foreign-table partition
(provided the foreign data wrapper supports tuple routing), they cannot be
moved from a foreign-table partition to another partition.
</para>
</refsect1>
<refsect1 id="sql-createforeigntable-examples">