diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 263e5024b62148863c139a523222af253871288a..e5856d9bfb4cea718b2e886313267f96f6cf7b92 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 1.73 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
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 463d2506c2e39554a8d8fb7ab600cfdb2905f9f2..3616275858392c370bdc5457557b48f701d08101 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 b6ac7786aa7a96d7a95912271cf1cbd9418a7094..0f7d8dc31fcae1c2640cb4e2d91c5059bc36f17d 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
@@ -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
*
diff --git a/src/include/optimizer/clauses.h b/src/include/optimizer/clauses.h
index d3fec6c444c02ab7c4f8db923937f461c894dbbc..0f452755e5ae51f77144a6d993b92a702ce35c21 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);