Avoid some table rewrites for ALTER TABLE .. SET DATA TYPE timestamp.

When the timezone is UTC, timestamptz and timestamp are binary coercible
in both directions.  See b8a18ad485 and
c22ecc6562 for the previous attempt in
this problem space.  Skip the table rewrite; for now, continue to
needlessly rewrite any index on an affected column.

Reviewed by Simon Riggs and Tom Lane.

Discussion: https://postgr.es/m/20190226061450.GA1665944@rfd.leadboat.com

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;
 	}

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
  */

src/include/utils/timestamp.h

@@ -104,4 +104,6 @@ extern int	date2isoweek(int year, int mon, int mday);
 extern int	date2isoyear(int year, int mon, int mday);
 extern int	date2isoyearday(int year, int mon, int mday);
 
+extern bool TimestampTimestampTzRequiresRewrite(void);
+
 #endif							/* TIMESTAMP_H */

src/test/regress/expected/event_trigger.out

@@ -435,7 +435,7 @@ END;
 $$;
 create event trigger no_rewrite_allowed on table_rewrite
   execute procedure test_evtrig_no_rewrite();
-create table rewriteme (id serial primary key, foo float);
+create table rewriteme (id serial primary key, foo float, bar timestamptz);
 insert into rewriteme
      select x * 1.001 from generate_series(1, 500) as t(x);
 alter table rewriteme alter column foo type numeric;
@@ -458,6 +458,15 @@ alter table rewriteme
 NOTICE:  Table 'rewriteme' is being rewritten (reason = 4)
 -- shouldn't trigger a table_rewrite event
 alter table rewriteme alter column foo type numeric(12,4);
+begin;
+set timezone to 'UTC';
+alter table rewriteme alter column bar type timestamp;
+set timezone to '0';
+alter table rewriteme alter column bar type timestamptz;
+set timezone to 'Europe/London';
+alter table rewriteme alter column bar type timestamp; -- does rewrite
+NOTICE:  Table 'rewriteme' is being rewritten (reason = 4)
+rollback;
 -- typed tables are rewritten when their type changes.  Don't emit table
 -- name, because firing order is not stable.
 CREATE OR REPLACE FUNCTION test_evtrig_no_rewrite() RETURNS event_trigger

src/test/regress/sql/event_trigger.sql

@@ -329,7 +329,7 @@ $$;
 create event trigger no_rewrite_allowed on table_rewrite
   execute procedure test_evtrig_no_rewrite();
 
-create table rewriteme (id serial primary key, foo float);
+create table rewriteme (id serial primary key, foo float, bar timestamptz);
 insert into rewriteme
      select x * 1.001 from generate_series(1, 500) as t(x);
 alter table rewriteme alter column foo type numeric;
@@ -352,6 +352,14 @@ alter table rewriteme
 
 -- shouldn't trigger a table_rewrite event
 alter table rewriteme alter column foo type numeric(12,4);
+begin;
+set timezone to 'UTC';
+alter table rewriteme alter column bar type timestamp;
+set timezone to '0';
+alter table rewriteme alter column bar type timestamptz;
+set timezone to 'Europe/London';
+alter table rewriteme alter column bar type timestamp; -- does rewrite
+rollback;
 
 -- typed tables are rewritten when their type changes.  Don't emit table
 -- name, because firing order is not stable.