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Commit36d442a

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Clarify that cross-row constraints are unsupported
Maybe we'll implement them later, or maybe not, but let's make the statuquo clear for now.Author: Lætitia Avrot, Patrick FrancelleReviewers: too many to listDiscussion:https://postgr.es/m/CAB_COdhUuzNFOJfc7SNNso5rOuVA3ui93KMVunEM8Yih+K5A6A@mail.gmail.com
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‎doc/src/sgml/ddl.sgml

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@@ -403,6 +403,33 @@ CREATE TABLE products (
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ensure that a column does not contain null values, the not-null
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constraint described in the next section can be used.
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</para>
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<note>
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<para>
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<productname>PostgreSQL</productname> does not support
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<literal>CHECK</literal> constraints that reference table data other than
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the new or updated row being checked. While a <literal>CHECK</literal>
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constraint that violates this rule may appear to work in simple
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tests, it cannot guarantee that the database will not reach a state
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in which the constraint condition is false (due to subsequent changes
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of the other row(s) involved). This would cause a database dump and
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reload to fail. The reload could fail even when the complete
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database state is consistent with the constraint, due to rows not
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being loaded in an order that will satisfy the constraint. If
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possible, use <literal>UNIQUE</literal>, <literal>EXCLUDE</literal>,
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or <literal>FOREIGN KEY</literal> constraints to express
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cross-row and cross-table restrictions.
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</para>
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<para>
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If what you desire is a one-time check against other rows at row
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insertion, rather than a continuously-maintained consistency
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guarantee, a custom <link linkend="triggers">trigger</link> can be used
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to implement that. (This approach avoids the dump/reload problem because
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<application>pg_dump</application> does not reinstall triggers until after
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reloading data, so that the check will not be enforced during a dump/reload.)
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</para>
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</note>
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</sect2>
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<sect2>

‎doc/src/sgml/ref/create_table.sgml

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@@ -754,7 +754,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
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<para>
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Currently, <literal>CHECK</literal> expressions cannot contain
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subqueries nor refer to variables other than columns of the
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current row. The system column <literal>tableoid</literal>
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current row (see <xref linkend="ddl-constraints-check-constraints"/>).
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The system column <literal>tableoid</literal>
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may be referenced, but not any other system column.
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</para>
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