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Doc: clarify description of degenerate NATURAL joins. 

Claiming that NATURAL JOIN is equivalent to CROSS JOIN when there are
no common column names is only strictly correct if it's an inner join;
you can't say e.g. CROSS LEFT JOIN.  Better to explain it as meaning
JOIN ON TRUE, instead.  Per a suggestion from David Johnston.

Discussion: https://postgr.es/m/CAKFQuwb+mYszQhDS9f_dqRrk1=Pe-S6D=XMkAXcDf4ykKPmgKQ@mail.gmail.com
This commit is contained in:
Tom Lane 2017-07-20 12:41:26 -04:00
parent eb145fdfea
commit ed3dc224e5
2 changed files with 7 additions and 4 deletions
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4
doc/src/sgml/queries.sgml
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@ -391,8 +391,8 @@ FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_r
consisting of all column names that appear in both
input tables. As with <literal>USING</>, these columns appear
only once in the output table. If there are no common
column names, <literal>NATURAL</literal> behaves like
<literal>CROSS JOIN</literal>.
column names, <literal>NATURAL JOIN</literal> behaves like
<literal>JOIN ... ON TRUE</literal>, producing a cross-product join.
</para>
<note>

7
doc/src/sgml/ref/select.sgml
View File

@ -603,9 +603,12 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
<varlistentry>
<term><literal>NATURAL</literal></term>
<listitem>
<para><literal>NATURAL</literal> is shorthand for a
<para>
<literal>NATURAL</literal> is shorthand for a
<literal>USING</> list that mentions all columns in the two
tables that have the same names.
tables that have matching names. If there are no common
column names, <literal>NATURAL</literal> is equivalent
to <literal>ON TRUE</>.
</para>
</listitem>
</varlistentry>