title: Does clustering an index in PostgreSQL take advantage of preordered data?
body: <p>I create a table like so:</p>
  
  <pre><code>SELECT t1.c1, t2.c2, t3.c3, *several more columns*
  INTO t4
  FROM t1
  INNER JOIN t2 ON t1.j2 = t2.j2
  INNER JOIN t3 ON t1.j3 = t3.j3;
  </code></pre>
  
  <p>Then, I create a clustered primary key:</p>
  
  <pre><code>ALTER TABLE t4 ADD CONSTRAINT pk_t4 PRIMARY KEY (c1, c2, c3);
  CLUSTER t4 USING pk_t4;
  </code></pre>
  
  <p>If I add an <code>ORDER BY c1, c2, c3</code> clause to the <code>SELECT INTO</code> query, will that speed up the clustering of the primary key?</p>