In ALTER COLUMN TYPE, strip any implicit coercion operations appearing

at the top level of the column's old default expression before adding
an implicit coercion to the new column type.  This seems to satisfy the
principle of least surprise, as per discussion of bug #1290.

doc/src/sgml/ref/alter_table.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.73 2004/07/11 23:13:51 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.74 2004/10/22 17:20:04 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -459,6 +459,22 @@ ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
     data.
    </para>
 
+   <para>
+    The <literal>USING</literal> option of <literal>ALTER TYPE</> can actually
+    specify any expression involving the old values of the row; that is, it
+    can refer to other columns as well as the one being converted.  This allows
+    very general conversions to be done with the <literal>ALTER TYPE</>
+    syntax.  Because of this flexibility, the <literal>USING</literal>
+    expression is not applied to the column's default value (if any); the
+    result might not be a constant expression as required for a default.
+    This means that when there is no implicit or assignment cast from old to
+    new type, <literal>ALTER TYPE</> may fail to convert the default even
+    though a <literal>USING</literal> clause is supplied.  In such cases,
+    drop the default with <literal>DROP DEFAULT</>, perform the <literal>ALTER
+    TYPE</>, and then use <literal>SET DEFAULT</> to add a suitable new
+    default.
+   </para>
+
    <para>
     If a table has any descendant tables, it is not permitted to add,
     rename, or change the type of a column in the parent table without doing

src/backend/commands/tablecmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.136 2004/10/21 21:33:59 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.137 2004/10/22 17:20:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4738,13 +4738,20 @@ ATExecAlterColumnType(AlteredTableInfo *tab, Relation rel,
 	 * changing the column type, because build_column_default itself will
 	 * try to coerce, and will not issue the error message we want if it
 	 * fails.)
+	 *
+	 * We remove any implicit coercion steps at the top level of the old
+	 * default expression; this has been agreed to satisfy the principle
+	 * of least surprise.  (The conversion to the new column type should
+	 * act like it started from what the user sees as the stored expression,
+	 * and the implicit coercions aren't going to be shown.)
 	 */
 	if (attTup->atthasdef)
 	{
 		defaultexpr = build_column_default(rel, attnum);
 		Assert(defaultexpr);
+		defaultexpr = strip_implicit_coercions(defaultexpr);
 		defaultexpr = coerce_to_target_type(NULL,		/* no UNKNOWN params */
-									  defaultexpr, exprType(defaultexpr),
+											defaultexpr, exprType(defaultexpr),
 											targettype, typename->typmod,
 											COERCION_ASSIGNMENT,
 											COERCE_IMPLICIT_CAST);

src/backend/optimizer/util/clauses.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.182 2004/10/07 18:38:49 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.183 2004/10/22 17:20:05 tgl Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -1021,6 +1021,41 @@ CommuteClause(OpExpr *clause)
 	lsecond(clause->args) = temp;
 }
 
+/*
+ * strip_implicit_coercions: remove implicit coercions at top level of tree
+ *
+ * Note: there isn't any useful thing we can do with a RowExpr here, so
+ * just return it unchanged, even if it's marked as an implicit coercion.
+ */
+Node *
+strip_implicit_coercions(Node *node)
+{
+	if (node == NULL)
+		return NULL;
+	if (IsA(node, FuncExpr))
+	{
+		FuncExpr   *f = (FuncExpr *) node;
+
+		if (f->funcformat == COERCE_IMPLICIT_CAST)
+			return strip_implicit_coercions(linitial(f->args));
+	}
+	else if (IsA(node, RelabelType))
+	{
+		RelabelType *r = (RelabelType *) node;
+
+		if (r->relabelformat == COERCE_IMPLICIT_CAST)
+			return strip_implicit_coercions((Node *) r->arg);
+	}
+	else if (IsA(node, CoerceToDomain))
+	{
+		CoerceToDomain *c = (CoerceToDomain *) node;
+
+		if (c->coercionformat == COERCE_IMPLICIT_CAST)
+			return strip_implicit_coercions((Node *) c->arg);
+	}
+	return node;
+}
+
 /*
  * set_coercionform_dontcare: set all CoercionForm fields to COERCE_DONTCARE
  *

src/include/optimizer/clauses.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/clauses.h,v 1.75 2004/08/29 04:13:07 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/clauses.h,v 1.76 2004/10/22 17:20:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -63,6 +63,8 @@ extern bool has_distinct_on_clause(Query *query);
 extern int	NumRelids(Node *clause);
 extern void CommuteClause(OpExpr *clause);
 
+extern Node *strip_implicit_coercions(Node *node);
+
 extern void set_coercionform_dontcare(Node *node);
 
 extern Node *eval_const_expressions(Node *node);