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Doc: clarify description of degenerate NATURAL joins.

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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>