diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index c753eb63007..17b0f3667da 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.87 2006/07/04 18:07:24 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.88 2006/08/03 20:57:06 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -741,6 +741,19 @@ ALTER TABLE foo
 </programlisting>
   </para>
 
+  <para>
+   The same, when the column has a default expression that won't automatically
+   cast to the new datatype:
+<programlisting>
+ALTER TABLE foo
+    ALTER COLUMN foo_timestamp DROP DEFAULT,
+    ALTER COLUMN foo_timestamp TYPE timestamp with time zone
+    USING
+        timestamp with time zone 'epoch' + foo_timestamp * interval '1 second',
+    ALTER COLUMN foo_timestamp SET DEFAULT now();
+</programlisting>
+  </para>
+
   <para>
    To rename an existing column:
 <programlisting>
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 3b408b411a6..cd4c4eb2304 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.198 2006/07/31 20:09:00 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.199 2006/08/03 20:57:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2000,7 +2000,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			ATSimplePermissions(rel, true);
 			ATSimpleRecursion(wqueue, rel, cmd, recurse);
 			/* No command-specific prep needed */
-			pass = AT_PASS_ADD_CONSTR;
+			pass = cmd->def ? AT_PASS_ADD_CONSTR : AT_PASS_DROP;
 			break;
 		case AT_DropNotNull:	/* ALTER COLUMN DROP NOT NULL */
 			ATSimplePermissions(rel, false);