Add transform functions for AT TIME ZONE.

This makes "ALTER TABLE tabname ALTER tscol TYPE ... USING tscol AT TIME
ZONE 'UTC'" skip rewriting the table when altering from "timestamp" to
"timestamptz" or vice versa.  While it would be nicer still to optimize
this in the absence of the USING clause given timezone==UTC, transform
functions must consult IMMUTABLE facts only.

1 files changed, 128 insertions, 0 deletions

diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 67e0cf95434..723c67087e5 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -27,6 +27,7 @@
 #include "funcapi.h"
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
+#include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
 #include "parser/scansup.h"
 #include "utils/array.h"
@@ -4874,6 +4875,87 @@ interval_part(PG_FUNCTION_ARGS)
 }
 
 
+/* timestamp_zone_transform()
+ * If the zone argument of a timestamp_zone() or timestamptz_zone() call is a
+ * plan-time constant denoting a zone equivalent to UTC, the call will always
+ * return its second argument unchanged.  Simplify the expression tree
+ * accordingly.  Civil time zones almost never qualify, because jurisdictions
+ * that follow UTC today have not done so continuously.
+ */
+Datum
+timestamp_zone_transform(PG_FUNCTION_ARGS)
+{
+	Node	   *func_node = (Node *) PG_GETARG_POINTER(0);
+	FuncExpr   *expr = (FuncExpr *) func_node;
+	Node	   *ret = NULL;
+	Node	   *zone_node;
+
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) == 2);
+
+	zone_node = (Node *) linitial(expr->args);
+
+	if (IsA(zone_node, Const) &&!((Const *) zone_node)->constisnull)
+	{
+		text	   *zone = DatumGetTextPP(((Const *) zone_node)->constvalue);
+		char		tzname[TZ_STRLEN_MAX + 1];
+		char	   *lowzone;
+		int			type,
+					abbrev_offset;
+		pg_tz	   *tzp;
+		bool		noop = false;
+
+		/*
+		 * If the timezone is forever UTC+0, the FuncExpr function call is a
+		 * no-op for all possible timestamps.  This passage mirrors code in
+		 * timestamp_zone().
+		 */
+		text_to_cstring_buffer(zone, tzname, sizeof(tzname));
+		lowzone = downcase_truncate_identifier(tzname,
+											   strlen(tzname),
+											   false);
+		type = DecodeTimezoneAbbrev(0, lowzone, &abbrev_offset, &tzp);
+		if (type == TZ || type == DTZ)
+			noop = (abbrev_offset == 0);
+		else if (type == DYNTZ)
+		{
+			/*
+			 * An abbreviation of a single-offset timezone ought not to be
+			 * configured as a DYNTZ, so don't bother checking.
+			 */
+		}
+		else
+		{
+			long		tzname_offset;
+
+			tzp = pg_tzset(tzname);
+			if (tzp && pg_get_timezone_offset(tzp, &tzname_offset))
+				noop = (tzname_offset == 0);
+		}
+
+		if (noop)
+		{
+			Node	   *timestamp = (Node *) lsecond(expr->args);
+
+			/* Strip any existing RelabelType node(s) */
+			while (timestamp && IsA(timestamp, RelabelType))
+				timestamp = (Node *) ((RelabelType *) timestamp)->arg;
+
+			/*
+			 * Replace the FuncExpr with its timestamp argument, relabeled as
+			 * though the function call had computed it.
+			 */
+			ret = (Node *) makeRelabelType((Expr *) timestamp,
+										   exprType(func_node),
+										   exprTypmod(func_node),
+										   exprCollation(func_node),
+										   COERCE_EXPLICIT_CAST);
+		}
+	}
+
+	PG_RETURN_POINTER(ret);
+}
+
 /*	timestamp_zone()
  *	Encode timestamp type with specified time zone.
  *	This function is just timestamp2timestamptz() except instead of
@@ -4963,6 +5045,52 @@ timestamp_zone(PG_FUNCTION_ARGS)
 	PG_RETURN_TIMESTAMPTZ(result);
 }
 
+/* timestamp_izone_transform()
+ * If we deduce at plan time that a particular timestamp_izone() or
+ * timestamptz_izone() call can only compute tz=0, the call will always return
+ * its second argument unchanged.  Simplify the expression tree accordingly.
+ */
+Datum
+timestamp_izone_transform(PG_FUNCTION_ARGS)
+{
+	Node	   *func_node = (Node *) PG_GETARG_POINTER(0);
+	FuncExpr   *expr = (FuncExpr *) func_node;
+	Node	   *ret = NULL;
+	Node	   *zone_node;
+
+	Assert(IsA(expr, FuncExpr));
+	Assert(list_length(expr->args) == 2);
+
+	zone_node = (Node *) linitial(expr->args);
+
+	if (IsA(zone_node, Const) &&!((Const *) zone_node)->constisnull)
+	{
+		Interval   *zone;
+
+		zone = DatumGetIntervalP(((Const *) zone_node)->constvalue);
+		if (zone->month == 0 && zone->day == 0 && zone->time == 0)
+		{
+			Node	   *timestamp = (Node *) lsecond(expr->args);
+
+			/* Strip any existing RelabelType node(s) */
+			while (timestamp && IsA(timestamp, RelabelType))
+				timestamp = (Node *) ((RelabelType *) timestamp)->arg;
+
+			/*
+			 * Replace the FuncExpr with its timestamp argument, relabeled as
+			 * though the function call had computed it.
+			 */
+			ret = (Node *) makeRelabelType((Expr *) timestamp,
+										   exprType(func_node),
+										   exprTypmod(func_node),
+										   exprCollation(func_node),
+										   COERCE_EXPLICIT_CAST);
+		}
+	}
+
+	PG_RETURN_POINTER(ret);
+}
+
 /* timestamp_izone()
  * Encode timestamp type with specified time interval as time zone.
  */