Fix extract epoch from interval calculation

The new numeric code for extract epoch from interval accidentally
truncated the DAYS_PER_YEAR value to an integer, leading to results
that mismatched the floating-point interval_part calculations.

The commit a2da77cd that introduced
this actually contains the regression test change that this reverts.
I suppose this was missed at the time.
Reported-by: Joseph Koshakow <koshy44@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/CAAvxfHd5n%3D13NYA2q_tUq%3D3%3DSuWU-CufmTf-Ozj%3DfrEgt7pXwQ%40mail.gmail.com

src/backend/utils/adt/timestamp.c

@@ -5287,10 +5287,16 @@ interval_part_common(PG_FUNCTION_ARGS, bool retnumeric)
 			int64		secs_from_day_month;
 			int64		val;
 
-			/* this always fits into int64 */
-			secs_from_day_month = ((int64) DAYS_PER_YEAR * (interval->month / MONTHS_PER_YEAR) +
-								   (int64) DAYS_PER_MONTH * (interval->month % MONTHS_PER_YEAR) +
-								   interval->day) * SECS_PER_DAY;
+			/*
+			 * To do this calculation in integer arithmetic even though
+			 * DAYS_PER_YEAR is fractional, multiply everything by 4 and then
+			 * divide by 4 again at the end.  This relies on DAYS_PER_YEAR
+			 * being a multiple of 0.25 and on SECS_PER_DAY being a multiple
+			 * of 4.
+			 */
+			secs_from_day_month = ((int64) (4 * DAYS_PER_YEAR) * (interval->month / MONTHS_PER_YEAR) +
+								   (int64) (4 * DAYS_PER_MONTH) * (interval->month % MONTHS_PER_YEAR) +
+								   (int64) 4 * interval->day) * (SECS_PER_DAY / 4);
 
 			/*---
 			 * result = secs_from_day_month + interval->time / 1'000'000

src/test/regress/expected/interval.out

@@ -953,11 +953,11 @@ SELECT f1,
  @ 1 min                       |           0 |       0.000 |   0.000000 |      1 |    0 |   0 |     0 |       1 |    0 |      0 |       0 |          0 |         60.000000
  @ 5 hours                     |           0 |       0.000 |   0.000000 |      0 |    5 |   0 |     0 |       1 |    0 |      0 |       0 |          0 |      18000.000000
  @ 10 days                     |           0 |       0.000 |   0.000000 |      0 |    0 |  10 |     0 |       1 |    0 |      0 |       0 |          0 |     864000.000000
- @ 34 years                    |           0 |       0.000 |   0.000000 |      0 |    0 |   0 |     0 |       1 |   34 |      3 |       0 |          0 | 1072224000.000000
+ @ 34 years                    |           0 |       0.000 |   0.000000 |      0 |    0 |   0 |     0 |       1 |   34 |      3 |       0 |          0 | 1072958400.000000
  @ 3 mons                      |           0 |       0.000 |   0.000000 |      0 |    0 |   0 |     3 |       2 |    0 |      0 |       0 |          0 |    7776000.000000
  @ 14 secs ago                 |   -14000000 |  -14000.000 | -14.000000 |      0 |    0 |   0 |     0 |       1 |    0 |      0 |       0 |          0 |        -14.000000
  @ 1 day 2 hours 3 mins 4 secs |     4000000 |    4000.000 |   4.000000 |      3 |    2 |   1 |     0 |       1 |    0 |      0 |       0 |          0 |      93784.000000
- @ 6 years                     |           0 |       0.000 |   0.000000 |      0 |    0 |   0 |     0 |       1 |    6 |      0 |       0 |          0 |  189216000.000000
+ @ 6 years                     |           0 |       0.000 |   0.000000 |      0 |    0 |   0 |     0 |       1 |    6 |      0 |       0 |          0 |  189345600.000000
  @ 5 mons                      |           0 |       0.000 |   0.000000 |      0 |    0 |   0 |     5 |       2 |    0 |      0 |       0 |          0 |   12960000.000000
  @ 5 mons 12 hours             |           0 |       0.000 |   0.000000 |      0 |   12 |   0 |     5 |       2 |    0 |      0 |       0 |          0 |   13003200.000000
 (10 rows)