*** a/src/backend/utils/adt/formatting.c
--- b/src/backend/utils/adt/formatting.c
***************
*** 59,64 ****
--- 59,65 ----
   * -----------------------------------------------------------------------
   */
  
+ /* XXX: DEBUG_TO_FROM_CHAR may access uninitialized memory */
  #ifdef DEBUG_TO_FROM_CHAR
  #define DEBUG_elog_output	DEBUG3
  #endif
*************** float8_to_char(PG_FUNCTION_ARGS)
*** 5427,5440 ****
  			Num.pre += Num.multi;
  		}
  		orgnum = (char *) palloc(MAXDOUBLEWIDTH + 1);
! 		len = snprintf(orgnum, MAXDOUBLEWIDTH + 1, "%.0f", fabs(val));
! 		if (Num.pre > len)
! 			plen = Num.pre - len;
! 		if (len >= DBL_DIG)
! 			Num.post = 0;
! 		else if (Num.post + len > DBL_DIG)
! 			Num.post = DBL_DIG - len;
! 		snprintf(orgnum, MAXDOUBLEWIDTH + 1, "%.*f", Num.post, val);
  
  		if (*orgnum == '-')
  		{						/* < 0 */
--- 5428,5439 ----
  			Num.pre += Num.multi;
  		}
  		orgnum = (char *) palloc(MAXDOUBLEWIDTH + 1);
! 
! 		len = snprintf(orgnum, MAXDOUBLEWIDTH + 1, "%.*f", Num.post, val);
! 
! 		/* Defensive */
! 		if (len >= MAXDOUBLEWIDTH + 1)
! 			elog(ERROR, "double precision for character conversion is too wide");
  
  		if (*orgnum == '-')
  		{						/* < 0 */
*** a/src/test/regress/expected/window.out
--- b/src/test/regress/expected/window.out
*************** SELECT to_char(SUM(n::float8) OVER (ORDE
*** 1765,1771 ****
    FROM (VALUES(1,1e20),(2,1)) n(i,n);
           to_char          
  --------------------------
!   100000000000000000000
                        1.0
  (2 rows)
  
--- 1765,1771 ----
    FROM (VALUES(1,1e20),(2,1)) n(i,n);
           to_char          
  --------------------------
!   100000000000000000000.0
                        1.0
  (2 rows)