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-rw-r--r--doc/src/sgml/ddl.sgml27
-rw-r--r--doc/src/sgml/ref/create_table.sgml3
2 files changed, 29 insertions, 1 deletions
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 61c4a254603..bfe89ef8ae0 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -403,6 +403,33 @@ CREATE TABLE products (
ensure that a column does not contain null values, the not-null
constraint described in the next section can be used.
</para>
+
+ <note>
+ <para>
+ <productname>PostgreSQL</productname> does not support
+ <literal>CHECK</literal> constraints that reference table data other than
+ the new or updated row being checked. While a <literal>CHECK</literal>
+ constraint that violates this rule may appear to work in simple
+ tests, it cannot guarantee that the database will not reach a state
+ in which the constraint condition is false (due to subsequent changes
+ of the other row(s) involved). This would cause a database dump and
+ reload to fail. The reload could fail even when the complete
+ database state is consistent with the constraint, due to rows not
+ being loaded in an order that will satisfy the constraint. If
+ possible, use <literal>UNIQUE</literal>, <literal>EXCLUDE</literal>,
+ or <literal>FOREIGN KEY</literal> constraints to express
+ cross-row and cross-table restrictions.
+ </para>
+
+ <para>
+ If what you desire is a one-time check against other rows at row
+ insertion, rather than a continuously-maintained consistency
+ guarantee, a custom <link linkend="triggers">trigger</link> can be used
+ to implement that. (This approach avoids the dump/reload problem because
+ <application>pg_dump</application> does not reinstall triggers until after
+ reloading data, so that the check will not be enforced during a dump/reload.)
+ </para>
+ </note>
</sect2>
<sect2>
diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 50d55970020..d3e33132f37 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -754,7 +754,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
<para>
Currently, <literal>CHECK</literal> expressions cannot contain
subqueries nor refer to variables other than columns of the
- current row. The system column <literal>tableoid</literal>
+ current row (see <xref linkend="ddl-constraints-check-constraints"/>).
+ The system column <literal>tableoid</literal>
may be referenced, but not any other system column.
</para>