title: Index spanning multiple tables in PostgreSQL
body: <p>Is it possible in PostgreSQL to place an index on an expression containing fields of multiple tables. So for example an index to speed up an query of the following form:</p>
  
  <pre><code>SELECT *, (table1.x + table2.x) AS z
  FROM table1
  INNER JOIN table2
  ON table1.id = table2.id
  ORDER BY z ASC
  </code></pre>