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POPostgreSQL: Auto-increment based on multi-column unique constraint
 

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    <p>One of my tables has the following definition:</p> <pre><code>CREATE TABLE incidents ( id serial NOT NULL, report integer NOT NULL, year integer NOT NULL, month integer NOT NULL, number integer NOT NULL, -- Report serial number for this period ... CONSTRAINT PRIMARY KEY (id), CONSTRAINT UNIQUE (report, year, month, number) ); </code></pre> <p>How would you go about incrementing the <code>number</code> column for every <code>report</code>, <code>year</code>, and <code>month</code> <em>independently</em>? I'd like to avoid creating a sequence or table for each (<code>report</code>, <code>year</code>, <code>month</code>) set.</p> <p>It would be nice if PostgreSQL supported incrementing "<a href="http://dev.mysql.com/doc/refman/5.1/en/example-auto-increment.html" rel="nofollow noreferrer">on a secondary column in a multiple-column index</a>" like MySQL's MyISAM tables, but I couldn't find a mention of such a feature in the <a href="http://www.postgresql.org/docs/8.2/static/datatype-numeric.html#DATATYPE-SERIAL" rel="nofollow noreferrer">manual</a>.</p> <p>An obvious solution is to select the current value <em>in the table</em> + 1, but this obviously is not safe for concurrent sessions. Maybe a pre-insert trigger would work, but are they guaranteed to be non-concurrent?</p> <p>Also note that I'm inserting incidents individually, so I can't use <a href="http://www.postgresql.org/docs/8.2/static/functions-srf.html" rel="nofollow noreferrer">generate_series</a> as suggested <a href="https://stackoverflow.com/questions/4005969/increment-field-with-not-null-and-unique-constraint-in-postgresql-8-3">elsewhere</a>.</p>
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