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Diffstat (limited to 'doc/src/sgml/ddl.sgml')
| -rw-r--r-- | doc/src/sgml/ddl.sgml | 27 |
1 files changed, 27 insertions, 0 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> |