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    <p>Rules by default <a href="http://www.postgresql.org/docs/current/static/sql-createrule.html" rel="noreferrer">add things to the current action</a>:</p> <blockquote> <p>Roughly speaking, a rule causes additional commands to be executed when a given command on a given table is executed.</p> </blockquote> <p>But an INSTEAD rule allows you to replace the action:</p> <blockquote> <p>Alternatively, an INSTEAD rule can replace a given command by another, or cause a command not to be executed at all.</p> </blockquote> <p>So, I think you want <a href="http://www.postgresql.org/docs/current/static/rules-update.html" rel="noreferrer">to specify INSTEAD</a>:</p> <pre><code>CREATE OR REPLACE RULE ignore_duplicate_inserts AS ON INSERT TO mytable WHERE (EXISTS ( SELECT mytable.id FROM mytable WHERE mytable.id = new.id)) DO INSTEAD NOTHING; </code></pre> <p>Without the INSTEAD, your rule is essentially saying "do the INSERT and then do nothing" when you want to say "instead of the INSERT, do nothing" and, AFAIK, the <code>DO INSTEAD NOTHING</code> will do that.</p> <p>I'm not an expert on PostgreSQL rules but I think adding the "INSTEAD" should work.</p> <p><strong>UPDATE</strong>: Thanks to araqnid <a href="http://www.postgresql.org/docs/9.0/interactive/sql-copy.html#AEN58860" rel="noreferrer">we know that</a>:</p> <blockquote> <p>COPY FROM will invoke any triggers and check constraints on the destination table. However, it will not invoke rules</p> </blockquote> <p>So a rule isn't going to work in this situation. However, triggers are fired during COPY FROM so you could write a BEFORE INSERT <a href="http://www.postgresql.org/docs/9.0/interactive/trigger-definition.html" rel="noreferrer">trigger that would return NULL</a> when it detected duplicate rows:</p> <blockquote> <p>It can return NULL to skip the operation for the current row. This instructs the executor to not perform the row-level operation that invoked the trigger (the insertion or modification of a particular table row).</p> </blockquote> <p>That said, I think you'd be better off with araqnid's "load it all into a temporary table, clean it up, and copy it to the final destination" would be a more sensible solution for a bulk loading operation like you have.</p>
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    1. POWhy doesn't this rule prevent duplicate key violations?
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    1. COThis would work for actual INSERT statements, but not COPY.
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    2. CO@araqnid: "COPY FROM will invoke any triggers and check constraints on the destination table. However, it will not invoke rules." You could use a BEFORE INSERT trigger and return NULL to skip the duplicates though. OTOH, triggers probably aren't the best tool for dealing with bulk data loading.
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