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Commit739b872

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Doc: extend warnings about collation-mismatch hazards in postgres_fdw.
Be a little more vocal about the risks of remote collations notmatching local ones. Actually fixing these risks seems hard,and I've given up on the idea that it might be back-patchable.So the best we can do for the back branches is add documentation.Per discussion of bug #16583 from Jiří Fejfar.Discussion:https://postgr.es/m/2438715.1632510693@sss.pgh.pa.us
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‎doc/src/sgml/postgres-fdw.sgml

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referenced columns of the remote table. Although <filename>postgres_fdw</filename>
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is currently rather forgiving about performing data type conversions at
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need, surprising semantic anomalies may arise when types or collations do
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not match, due to the remote server interpreting<literal>WHERE</literal> clauses
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slightlydifferently from the local server.
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not match, due to the remote server interpretingquery conditions
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differently from the local server.
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</para>
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<para>
@@ -423,6 +423,17 @@ OPTIONS (ADD password_required 'false');
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need to turn this off if the remote server has a different set of
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collation names than the local server does, which is likely to be the
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case if it's running on a different operating system.
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If you do so, however, there is a very severe risk that the imported
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table columns' collations will not match the underlying data, resulting
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in anomalous query behavior.
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</para>
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<para>
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Even when this parameter is set to <literal>true</literal>, importing
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columns whose collation is the remote server's default can be risky.
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They will be imported with <literal>COLLATE "default"</literal>, which
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will select the local server's default collation, which could be
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different.
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</para>
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</listitem>
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</varlistentry>

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