Index: src/backend/utils/adt/timestamp.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/utils/adt/timestamp.c,v
retrieving revision 1.165
diff -c -c -r1.165 timestamp.c
*** src/backend/utils/adt/timestamp.c	13 Jul 2006 16:49:16 -0000	1.165
--- src/backend/utils/adt/timestamp.c	30 Aug 2006 17:08:12 -0000
***************
*** 2496,2501 ****
--- 2496,2502 ----
  				day_remainder,
  				month_remainder_days;
  	Interval   *result;
+ 	int32		orig_month = span->month;
  
  	result = (Interval *) palloc(sizeof(Interval));
  
***************
*** 2516,2523 ****
  	 * using justify_hours and/or justify_days.
  	 */
  
! 	/* fractional months full days into days */
! 	month_remainder_days = month_remainder * DAYS_PER_MONTH;
  	result->day += (int32) month_remainder_days;
  	/* fractional months partial days into time */
  	day_remainder += month_remainder_days - (int32) month_remainder_days;
--- 2517,2533 ----
  	 * using justify_hours and/or justify_days.
  	 */
  
! 	/*
! 	 *	Fractional months full days into days.
! 	 *
! 	 *	The remainders suffer from float rounding, so instead of
! 	 *	doing the computation using just the remainder, we calculate
! 	 *	the total number of days and subtract.  Specifically, we are
! 	 *	multipling by DAYS_PER_MONTH before dividing by factor.
! 	 *	This greatly reduces rounding errors.
! 	 */
! 	month_remainder_days = (orig_month * DAYS_PER_MONTH) * factor -
! 			result->month * DAYS_PER_MONTH;
  	result->day += (int32) month_remainder_days;
  	/* fractional months partial days into time */
  	day_remainder += month_remainder_days - (int32) month_remainder_days;
***************
*** 2550,2556 ****
  				day_remainder,
  				month_remainder_days;
  	Interval   *result;
! 
  	result = (Interval *) palloc(sizeof(Interval));
  
  	if (factor == 0.0)
--- 2560,2567 ----
  				day_remainder,
  				month_remainder_days;
  	Interval   *result;
! 	int32		orig_month = span->month;
! 	
  	result = (Interval *) palloc(sizeof(Interval));
  
  	if (factor == 0.0)
***************
*** 2566,2576 ****
  	day_remainder -= result->day;
  
  	/*
! 	 * Handle any fractional parts the same way as in interval_mul.
  	 */
! 
! 	/* fractional months full days into days */
! 	month_remainder_days = month_remainder * DAYS_PER_MONTH;
  	result->day += (int32) month_remainder_days;
  	/* fractional months partial days into time */
  	day_remainder += month_remainder_days - (int32) month_remainder_days;
--- 2577,2587 ----
  	day_remainder -= result->day;
  
  	/*
! 	 *	Fractional months full days into days.  See comment in 
! 	 *	interval_mul().
  	 */
! 	month_remainder_days = (orig_month * DAYS_PER_MONTH) / factor -
! 			result->month * DAYS_PER_MONTH;
  	result->day += (int32) month_remainder_days;
  	/* fractional months partial days into time */
  	day_remainder += month_remainder_days - (int32) month_remainder_days;
Index: src/test/regress/expected/interval.out
===================================================================
RCS file: /cvsroot/pgsql/src/test/regress/expected/interval.out,v
retrieving revision 1.15
diff -c -c -r1.15 interval.out
*** src/test/regress/expected/interval.out	6 Mar 2006 22:49:17 -0000	1.15
--- src/test/regress/expected/interval.out	30 Aug 2006 17:08:16 -0000
***************
*** 218,224 ****
  select avg(f1) from interval_tbl;
                         avg                       
  -------------------------------------------------
!  @ 4 years 1 mon 9 days 28 hours 18 mins 23 secs
  (1 row)
  
  -- test long interval input
--- 218,224 ----
  select avg(f1) from interval_tbl;
                         avg                       
  -------------------------------------------------
!  @ 4 years 1 mon 10 days 4 hours 18 mins 23 secs
  (1 row)
  
  -- test long interval input