interval: round values when spilling to months

Previously spilled units greater than months were truncated to months.
Also document the spill behavior.

Reported-by: Bryn Llewelly

Discussion: https://postgr.es/m/BDAE4B56-3337-45A2-AC8A-30593849D6C0@yugabyte.com

Backpatch-through: master

doc/src/sgml/datatype.sgml

@@ -2840,15 +2840,18 @@ P <optional> <replaceable>years</replaceable>-<replaceable>months</replaceable>-
     </para>
 
     <para>
-     In the verbose input format, and in some fields of the more compact
+     Field values can have fractional parts:  for example, <literal>'1.5
-     input formats, field values can have fractional parts; for example
+     weeks'</literal> or <literal>'01:02:03.45'</literal>.  However,
-     <literal>'1.5 week'</literal> or <literal>'01:02:03.45'</literal>.  Such input is
+     because interval internally stores only three integer units (months,
-     converted to the appropriate number of months, days, and seconds
+     days, microseconds), fractional units must be spilled to smaller
-     for storage.  When this would result in a fractional number of
+     units.  Fractional parts of units greater than months is rounded to
-     months or days, the fraction is added to the lower-order fields
+     be an integer number of months, e.g. <literal>'1.5 years'</literal>
-     using the conversion factors 1 month = 30 days and 1 day = 24 hours.
+     becomes <literal>'1 year 6 mons'</literal>.  Fractional parts of
-     For example, <literal>'1.5 month'</literal> becomes 1 month and 15 days.
+     weeks and days are computed to be an integer number of days and
-     Only seconds will ever be shown as fractional on output.
+     microseconds, assuming 30 days per month and 24 hours per day, e.g.,
+     <literal>'1.75 months'</literal> becomes <literal>1 mon 22 days
+     12:00:00</literal>.  Only seconds will ever be shown as fractional
+     on output.
     </para>
 
     <para>
@@ -2892,10 +2895,10 @@ P <optional> <replaceable>years</replaceable>-<replaceable>months</replaceable>-
 
     <para>
      Internally <type>interval</type> values are stored as months, days,
-     and seconds. This is done because the number of days in a month
+     and microseconds. This is done because the number of days in a month
      varies, and a day can have 23 or 25 hours if a daylight savings
      time adjustment is involved.  The months and days fields are integers
-     while the seconds field can store fractions.  Because intervals are
+     while the microseconds field can store fractional seconds.  Because intervals are
      usually created from constant strings or <type>timestamp</type> subtraction,
      this storage method works well in most cases, but can cause unexpected
      results:

src/backend/utils/adt/datetime.c

@@ -3306,29 +3306,25 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 
 					case DTK_YEAR:
 						tm->tm_year += val;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
-							tm->tm_mon += fval * MONTHS_PER_YEAR;
 						tmask = DTK_M(YEAR);
 						break;
 
 					case DTK_DECADE:
 						tm->tm_year += val * 10;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR * 10);
-							tm->tm_mon += fval * MONTHS_PER_YEAR * 10;
 						tmask = DTK_M(DECADE);
 						break;
 
 					case DTK_CENTURY:
 						tm->tm_year += val * 100;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR * 100);
-							tm->tm_mon += fval * MONTHS_PER_YEAR * 100;
 						tmask = DTK_M(CENTURY);
 						break;
 
 					case DTK_MILLENNIUM:
 						tm->tm_year += val * 1000;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR * 1000);
-							tm->tm_mon += fval * MONTHS_PER_YEAR * 1000;
 						tmask = DTK_M(MILLENNIUM);
 						break;
 
@@ -3565,7 +3561,7 @@ DecodeISO8601Interval(char *str,
 			{
 				case 'Y':
 					tm->tm_year += val;
-					tm->tm_mon += (fval * MONTHS_PER_YEAR);
+					tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
 					break;
 				case 'M':
 					tm->tm_mon += val;
@@ -3601,7 +3597,7 @@ DecodeISO8601Interval(char *str,
 						return DTERR_BAD_FORMAT;
 
 					tm->tm_year += val;
-					tm->tm_mon += (fval * MONTHS_PER_YEAR);
+					tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
 					if (unit == '\0')
 						return 0;
 					if (unit == 'T')

src/interfaces/ecpg/pgtypeslib/interval.c

@@ -155,7 +155,7 @@ DecodeISO8601Interval(char *str,
 			{
 				case 'Y':
 					tm->tm_year += val;
-					tm->tm_mon += (fval * MONTHS_PER_YEAR);
+					tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
 					break;
 				case 'M':
 					tm->tm_mon += val;
@@ -191,7 +191,7 @@ DecodeISO8601Interval(char *str,
 						return DTERR_BAD_FORMAT;
 
 					tm->tm_year += val;
-					tm->tm_mon += (fval * MONTHS_PER_YEAR);
+					tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
 					if (unit == '\0')
 						return 0;
 					if (unit == 'T')
@@ -528,29 +528,25 @@ DecodeInterval(char **field, int *ftype, int nf,	/* int range, */
 
 					case DTK_YEAR:
 						tm->tm_year += val;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR);
-							tm->tm_mon += fval * MONTHS_PER_YEAR;
 						tmask = (fmask & DTK_M(YEAR)) ? 0 : DTK_M(YEAR);
 						break;
 
 					case DTK_DECADE:
 						tm->tm_year += val * 10;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR * 10);
-							tm->tm_mon += fval * MONTHS_PER_YEAR * 10;
 						tmask = (fmask & DTK_M(YEAR)) ? 0 : DTK_M(YEAR);
 						break;
 
 					case DTK_CENTURY:
 						tm->tm_year += val * 100;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR * 100);
-							tm->tm_mon += fval * MONTHS_PER_YEAR * 100;
 						tmask = (fmask & DTK_M(YEAR)) ? 0 : DTK_M(YEAR);
 						break;
 
 					case DTK_MILLENNIUM:
 						tm->tm_year += val * 1000;
-						if (fval != 0)
+						tm->tm_mon += rint(fval * MONTHS_PER_YEAR * 1000);
-							tm->tm_mon += fval * MONTHS_PER_YEAR * 1000;
 						tmask = (fmask & DTK_M(YEAR)) ? 0 : DTK_M(YEAR);
 						break;