Improve behavior of date_bin with origin in the future

Currently, when the origin is after the input, the result is the
timestamp at the end of the bin, rather than the beginning as
expected.  This puts the result consistently at the beginning of the
bin.

Author: John Naylor <john.naylor@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/CAFBsxsGjLDxQofRfH+d4KSAXxPf3MMevUG7s6EDfdBOvHLDLjw@mail.gmail.com

src/backend/utils/adt/timestamp.c

@@ -3846,6 +3846,13 @@ timestamp_bin(PG_FUNCTION_ARGS)
 	tm_diff = timestamp - origin;
 	tm_delta = tm_diff - tm_diff % stride_usecs;
 
+	/*
+	 * Make sure the returned timestamp is at the start of the bin,
+	 * even if the origin is in the future.
+	 */
+	if (origin > timestamp && stride_usecs > 1)
+		tm_delta -= stride_usecs;
+
 	result = origin + tm_delta;
 
 	PG_RETURN_TIMESTAMP(result);
@@ -4017,6 +4024,13 @@ timestamptz_bin(PG_FUNCTION_ARGS)
 	tm_diff = timestamp - origin;
 	tm_delta = tm_diff - tm_diff % stride_usecs;
 
+	/*
+	 * Make sure the returned timestamp is at the start of the bin,
+	 * even if the origin is in the future.
+	 */
+	if (origin > timestamp && stride_usecs > 1)
+		tm_delta -= stride_usecs;
+
 	result = origin + tm_delta;
 
 	PG_RETURN_TIMESTAMPTZ(result);

src/test/regress/expected/timestamp.out

@@ -609,6 +609,60 @@ FROM (
  microsecond | 1 us     | t
 (7 rows)
 
+-- case 3: AD dates, origin > input
+SELECT
+  str,
+  interval,
+  date_trunc(str, ts) = date_bin(interval::interval, ts, timestamp '2020-03-02') AS equal
+FROM (
+  VALUES
+  ('week', '7 d'),
+  ('day', '1 d'),
+  ('hour', '1 h'),
+  ('minute', '1 m'),
+  ('second', '1 s'),
+  ('millisecond', '1 ms'),
+  ('microsecond', '1 us')
+) intervals (str, interval),
+(VALUES (timestamp '2020-02-29 15:44:17.71393')) ts (ts);
+     str     | interval | equal 
+-------------+----------+-------
+ week        | 7 d      | t
+ day         | 1 d      | t
+ hour        | 1 h      | t
+ minute      | 1 m      | t
+ second      | 1 s      | t
+ millisecond | 1 ms     | t
+ microsecond | 1 us     | t
+(7 rows)
+
+-- case 4: BC dates, origin > input
+SELECT
+  str,
+  interval,
+  date_trunc(str, ts) = date_bin(interval::interval, ts, timestamp '0055-06-17 BC') AS equal
+FROM (
+  VALUES
+  ('week', '7 d'),
+  ('day', '1 d'),
+  ('hour', '1 h'),
+  ('minute', '1 m'),
+  ('second', '1 s'),
+  ('millisecond', '1 ms'),
+  ('microsecond', '1 us')
+) intervals (str, interval),
+(VALUES (timestamp '0055-6-10 15:44:17.71393 BC')) ts (ts);
+     str     | interval | equal 
+-------------+----------+-------
+ week        | 7 d      | t
+ day         | 1 d      | t
+ hour        | 1 h      | t
+ minute      | 1 m      | t
+ second      | 1 s      | t
+ millisecond | 1 ms     | t
+ microsecond | 1 us     | t
+(7 rows)
+
 -- bin timestamps into arbitrary intervals
 SELECT
   interval,

src/test/regress/sql/timestamp.sql

@@ -203,6 +203,40 @@ FROM (
 ) intervals (str, interval),
 (VALUES (timestamp '0055-6-10 15:44:17.71393 BC')) ts (ts);
 
+-- case 3: AD dates, origin > input
+SELECT
+  str,
+  interval,
+  date_trunc(str, ts) = date_bin(interval::interval, ts, timestamp '2020-03-02') AS equal
+FROM (
+  VALUES
+  ('week', '7 d'),
+  ('day', '1 d'),
+  ('hour', '1 h'),
+  ('minute', '1 m'),
+  ('second', '1 s'),
+  ('millisecond', '1 ms'),
+  ('microsecond', '1 us')
+) intervals (str, interval),
+(VALUES (timestamp '2020-02-29 15:44:17.71393')) ts (ts);
+
+-- case 4: BC dates, origin > input
+SELECT
+  str,
+  interval,
+  date_trunc(str, ts) = date_bin(interval::interval, ts, timestamp '0055-06-17 BC') AS equal
+FROM (
+  VALUES
+  ('week', '7 d'),
+  ('day', '1 d'),
+  ('hour', '1 h'),
+  ('minute', '1 m'),
+  ('second', '1 s'),
+  ('millisecond', '1 ms'),
+  ('microsecond', '1 us')
+) intervals (str, interval),
+(VALUES (timestamp '0055-6-10 15:44:17.71393 BC')) ts (ts);
+
 -- bin timestamps into arbitrary intervals
 SELECT
   interval,