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.

3 files changed, 48 insertions, 4 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 463d2506c2e..36162758583 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 1.136 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);
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index b6ac7786aa7..0f7d8dc31fc 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/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
@@ -1022,6 +1022,41 @@ CommuteClause(OpExpr *clause)
 }
 
 /*
+ * 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
  *
  * This is used to make index expressions and index predicates more easily
diff --git a/src/include/optimizer/clauses.h b/src/include/optimizer/clauses.h
index d3fec6c444c..0f452755e5a 100644
--- a/src/include/optimizer/clauses.h
+++ b/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);