 <p>zerkms's answer cuts to the gist of what you actually need to know, and is correct. Transaction isolation will make sure you're not affected by concurrency issues in this simple case (but note, it's not a magic solution to all concurrency).</p> <p>To answer your question literally as written: they're actually interleaved.</p> <p>If you write:</p> <pre><code>SELECT SUM(field1)+SUM(field2)+SUM(field3)-SUM(field4) FROM mytable; </code></pre> <p>then Pg reads <code>mytable</code> once. It reads each row and feeds the <code>mytable.field1</code> to the first sum, <code>mytable.field2</code> into the second sum, etc. Then it repeats for each row.</p> <p>So all the aggregation is done in parallel (though not using multiple CPUs).</p>