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Clarify that cross-row constraints are unsupported

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Maybe we'll implement them later, or maybe not, but let's make the statu
quo clear for now.

Author: Lætitia Avrot, Patrick Francelle
Reviewers: too many to list
Discussion: https://postgr.es/m/CAB_COdhUuzNFOJfc7SNNso5rOuVA3ui93KMVunEM8Yih+K5A6A@mail.gmail.com
This commit is contained in:
Alvaro Herrera 2018-11-26 12:27:07 -03:00
parent 664f01b613
commit 36d442a25a
2 changed files with 29 additions and 1 deletions
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27
doc/src/sgml/ddl.sgml
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@ -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>

3
doc/src/sgml/ref/create_table.sgml
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@ -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>