2 files changed, 44 insertions, 5 deletions
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 93f13a4778c..59341e2a40f 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -96,6 +96,7 @@
 #include "utils/ruleutils.h"
 #include "utils/snapmgr.h"
 #include "utils/syscache.h"
+#include "utils/timestamp.h"
 #include "utils/typcache.h"
 
 
@@ -9678,11 +9679,15 @@ ATPrepAlterColumnType(List **wqueue,
  * When the data type of a column is changed, a rewrite might not be required
  * if the new type is sufficiently identical to the old one, and the USING
  * clause isn't trying to insert some other value.  It's safe to skip the
- * rewrite if the old type is binary coercible to the new type, or if the
- * new type is an unconstrained domain over the old type.  In the case of a
- * constrained domain, we could get by with scanning the table and checking
- * the constraint rather than actually rewriting it, but we don't currently
- * try to do that.
+ * rewrite in these cases:
+ *
+ * - the old type is binary coercible to the new type
+ * - the new type is an unconstrained domain over the old type
+ * - {NEW,OLD} or {OLD,NEW} is {timestamptz,timestamp} and the timezone is UTC
+ *
+ * In the case of a constrained domain, we could get by with scanning the
+ * table and checking the constraint rather than actually rewriting it, but we
+ * don't currently try to do that.
  */
 static bool
 ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
@@ -9704,6 +9709,23 @@ ATColumnChangeRequiresRewrite(Node *expr, AttrNumber varattno)
 				return true;
 			expr = (Node *) d->arg;
 		}
+		else if (IsA(expr, FuncExpr))
+		{
+			FuncExpr   *f = (FuncExpr *) expr;
+
+			switch (f->funcid)
+			{
+				case F_TIMESTAMPTZ_TIMESTAMP:
+				case F_TIMESTAMP_TIMESTAMPTZ:
+					if (TimestampTimestampTzRequiresRewrite())
+						return true;
+					else
+						expr = linitial(f->args);
+					break;
+				default:
+					return true;
+			}
+		}
 		else
 			return true;
 	}
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index e0ef2f78616..1b0effa9249 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -5168,6 +5168,23 @@ timestamp_izone(PG_FUNCTION_ARGS)
 	PG_RETURN_TIMESTAMPTZ(result);
 }								/* timestamp_izone() */
 
+/* TimestampTimestampTzRequiresRewrite()
+ *
+ * Returns false if the TimeZone GUC setting causes timestamp_timestamptz and
+ * timestamptz_timestamp to be no-ops, where the return value has the same
+ * bits as the argument.  Since project convention is to assume a GUC changes
+ * no more often than STABLE functions change, the answer is valid that long.
+ */
+bool
+TimestampTimestampTzRequiresRewrite(void)
+{
+	long		offset;
+
+	if (pg_get_timezone_offset(session_timezone, &offset) && offset == 0)
+		PG_RETURN_BOOL(false);
+	PG_RETURN_BOOL(true);
+}
+
 /* timestamp_timestamptz()
  * Convert local timestamp to timestamp at GMT
  */