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 03:45:41 -0000
***************
*** 2518,2523 ****
--- 2518,2532 ----
  
  	/* fractional months full days into days */
  	month_remainder_days = month_remainder * DAYS_PER_MONTH;
+ 	/*
+ 	 *	The remainders suffer from float rounding, so if they are
+ 	 *	within 1us of an integer, we round them to integers.
+ 	 *	It seems doing two multiplies is causing the imprecision.
+ 	 */
+ 	if (month_remainder_days != (int32)month_remainder_days &&
+ 		TSROUND(month_remainder_days * SECS_PER_DAY) ==
+ 		rint(month_remainder_days * SECS_PER_DAY))
+ 		month_remainder_days = rint(month_remainder_days);
  	result->day += (int32) month_remainder_days;
  	/* fractional months partial days into time */
  	day_remainder += month_remainder_days - (int32) month_remainder_days;
***************
*** 2571,2576 ****
--- 2580,2595 ----
  
  	/* fractional months full days into days */
  	month_remainder_days = month_remainder * DAYS_PER_MONTH;
+ 	/*
+ 	 *	The remainders suffer from float rounding, so if they are
+ 	 *	within 1us of an integer, we round them to integers.
+ 	 *	It seems doing a division and multiplies is causing the
+ 	 *	imprecision.
+ 	 */
+ 	if (month_remainder_days != (int32)month_remainder_days &&
+ 		TSROUND(month_remainder_days * SECS_PER_DAY) ==
+ 		rint(month_remainder_days * SECS_PER_DAY))
+ 		month_remainder_days = rint(month_remainder_days);
  	result->day += (int32) month_remainder_days;
  	/* fractional months partial days into time */
  	day_remainder += month_remainder_days - (int32) month_remainder_days;
Index: src/include/utils/timestamp.h
===================================================================
RCS file: /cvsroot/pgsql/src/include/utils/timestamp.h,v
retrieving revision 1.62
diff -c -c -r1.62 timestamp.h
*** src/include/utils/timestamp.h	13 Jul 2006 16:49:20 -0000	1.62
--- src/include/utils/timestamp.h	30 Aug 2006 03:45:42 -0000
***************
*** 161,171 ****
  
  typedef double fsec_t;
  
! /* round off to MAX_TIMESTAMP_PRECISION decimal places */
! /* note: this is also used for rounding off intervals */
  #define TS_PREC_INV 1000000.0
  #define TSROUND(j) (rint(((double) (j)) * TS_PREC_INV) / TS_PREC_INV)
- #endif
  
  #define TIMESTAMP_MASK(b) (1 << (b))
  #define INTERVAL_MASK(b) (1 << (b))
--- 161,175 ----
  
  typedef double fsec_t;
  
! #endif
! 
! /*
!  *	Round off to MAX_TIMESTAMP_PRECISION decimal places.
!  *	This is also used for rounding off intervals, and
!  *	for rounding interval multiplication/division calculations.
!  */
  #define TS_PREC_INV 1000000.0
  #define TSROUND(j) (rint(((double) (j)) * TS_PREC_INV) / TS_PREC_INV)
  
  #define TIMESTAMP_MASK(b) (1 << (b))
  #define INTERVAL_MASK(b) (1 << (b))
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 03:45:43 -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