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Re: Typo in doc or wrong EXCLUDE implementation - Mailing list pgsql-hackers

From David G. Johnston
Subject Re: Typo in doc or wrong EXCLUDE implementation
Date
Msg-id CAKFQuwYCGDrazjV0ME_X81nchdj+b0WSJNwNQnNqYrThVUA41Q@mail.gmail.com
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In response to Re: Typo in doc or wrong EXCLUDE implementation  (Tom Lane <tgl@sss.pgh.pa.us>)
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On Thu, Aug 9, 2018 at 12:31 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
I think the OP is reading "equivalent" literally, as meaning that
an EXCLUDE with operators that act like equality is treated as being
the same as UNIQUE for *every* purpose.  We're not going there, IMO,
so probably we need to tweak the doc wording a little.  Perhaps
writing "functionally equivalent" would be better?  Or instead of
"is equivalent to", write "imposes the same restriction as"?

Maybe something like:

diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index d936de3f23..7c31fe853b 100644
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -928,12 +928,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       The <literal>EXCLUDE</literal> clause defines an exclusion
       constraint, which guarantees that if
       any two rows are compared on the specified column(s) or
-      expression(s) using the specified operator(s), not all of these
-      comparisons will return <literal>TRUE</literal>.  If all of the
-      specified operators test for equality, this is equivalent to a
-      <literal>UNIQUE</literal> constraint, although an ordinary unique constraint
-      will be faster.  However, exclusion constraints can specify
-      constraints that are more general than simple equality.
+      expression(s) using the specified operator(s), at least one of the
+      comparisons will return <literal>FALSE<literal/>.
+      Exclusion constraints can (and should) be used to specify
+      expressions that do not involve simple equality.
       For example, you can specify a constraint that
       no two rows in the table contain overlapping circles
       (see <xref linkend="datatype-geometric"/>) by using the
@@ -968,6 +966,14 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       exclusion constraint on a subset of the table; internally this creates a
       partial index. Note that parentheses are required around the predicate.
      </para>
+
+     <para>
+      <productname>PostgreSQL</productname> does not consider an exclusion
+      constraint to be a valid unique constraint for purposes of determining the
+      validity of a foreign key constraint.  For this reason, in addition to performance,
+      an exclusion constraint defined using only equality operators should be defined
+      as a <literal>UNIQUE<literal/> constraint.
+     </para>
     </listitem>
    </varlistentry>
 

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