From e70ec8230a2c0e7363bb7abf4d55dddbdec89fe1 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Mon, 25 May 2015 14:12:51 -0400
Subject: [PATCH] Explain CHECK constraint handling in postgres_fdw's IMPORT
FOREIGN SCHEMA.
The existing documentation could easily be misinterpreted, and it failed to
explain the inconsistent-evaluation hazard that deterred us from supporting
automatic importing of check constraints. Revise it.
Etsuro Fujita, further expanded by me
---
doc/src/sgml/postgres-fdw.sgml | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
diff --git a/doc/src/sgml/postgres-fdw.sgml b/doc/src/sgml/postgres-fdw.sgml
index 1079140de28..14b12e37dc6 100644
--- a/doc/src/sgml/postgres-fdw.sgml
+++ b/doc/src/sgml/postgres-fdw.sgml
@@ -309,8 +309,8 @@
using <xref linkend="sql-importforeignschema">. This command creates
foreign table definitions on the local server that match tables or
views present on the remote server. If the remote tables to be imported
- have columns of user-defined data types, the local server must have types
- of the same names.
+ have columns of user-defined data types, the local server must have
+ compatible types of the same names.
</para>
<para>
@@ -361,9 +361,16 @@
<para>
Note that constraints other than <literal>NOT NULL</> will never be
- imported from the remote tables, since <productname>PostgreSQL</>
- does not support any other type of constraint on a foreign table.
- Checking other types of constraints is always left to the remote server.
+ imported from the remote tables. Although <productname>PostgreSQL</>
+ does support <literal>CHECK</> constraints on foreign tables, there is no
+ provision for importing them automatically, because of the risk that a
+ constraint expression could evaluate differently on the local and remote
+ servers. Any such inconsistency in the behavior of a <literal>CHECK</>
+ constraint could lead to hard-to-detect errors in query optimization.
+ So if you wish to import <literal>CHECK</> constraints, you must do so
+ manually, and you should verify the semantics of each one carefully.
+ For more detail about the treatment of <literal>CHECK</> constraints on
+ foreign tables, see <xref linkend="sql-createforeigntable">.
</para>
</sect3>
</sect2>
--
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