Properly round months into days and into seconds for interval

multiplication/division queries like select '41 mon 10:00:00'::interval
/ 10 as "pos".

Report from Michael Glaesemann

diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 73f341a0355f42243560db56372cfe41ce7cdf1a..35886fd200e58e53573a6684599ebc2a7d33be06 100644 (file)
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.165 2006/07/13 16:49:16 momjian Exp $
+ *       $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.166 2006/09/03 03:34:04 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2492,19 +2492,14 @@ interval_mul(PG_FUNCTION_ARGS)
 {
        Interval   *span = PG_GETARG_INTERVAL_P(0);
        float8          factor = PG_GETARG_FLOAT8(1);
-       double          month_remainder,
-                               day_remainder,
-                               month_remainder_days;
+       double          month_remainder_days, sec_remainder;
+       int32           orig_month = span->month, orig_day = span->day;
        Interval   *result;
 
        result = (Interval *) palloc(sizeof(Interval));
 
-       month_remainder = span->month * factor;
-       day_remainder = span->day * factor;
-       result->month = (int32) month_remainder;
-       result->day = (int32) day_remainder;
-       month_remainder -= result->month;
-       day_remainder -= result->day;
+       result->month = (int32) (span->month * factor);
+       result->day = (int32) (span->day * factor);
 
        /*
         * The above correctly handles the whole-number part of the month and day
@@ -2516,16 +2511,31 @@ interval_mul(PG_FUNCTION_ARGS)
         * 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;
+       /*
+        *      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 * (double)DAYS_PER_MONTH) * factor -
+                       result->month * (double)DAYS_PER_MONTH;
+       sec_remainder = (orig_day * (double)SECS_PER_DAY) * factor -
+                       result->day * (double)SECS_PER_DAY +
+                       (month_remainder_days - (int32) month_remainder_days) * SECS_PER_DAY;
 
+       /* cascade units down */
+       result->day += (int32) month_remainder_days;
 #ifdef HAVE_INT64_TIMESTAMP
-       result->time = rint(span->time * factor + day_remainder * USECS_PER_DAY);
+       result->time = rint(span->time * factor + sec_remainder * USECS_PER_SEC);
 #else
-       result->time = span->time * factor + day_remainder * SECS_PER_DAY;
+       /*
+        *      TSROUND() needed to prevent -146:23:60.00 output on PowerPC for
+        *      SELECT interval '-41 mon -12 days -360:00' * 0.3;
+        */
+       result->time = span->time * factor + TSROUND(sec_remainder);
 #endif
 
        PG_RETURN_INTERVAL_P(result);
@@ -2546,11 +2556,10 @@ interval_div(PG_FUNCTION_ARGS)
 {
        Interval   *span = PG_GETARG_INTERVAL_P(0);
        float8          factor = PG_GETARG_FLOAT8(1);
-       double          month_remainder,
-                               day_remainder,
-                               month_remainder_days;
+       double          month_remainder_days, sec_remainder;
+       int32           orig_month = span->month, orig_day = span->day;
        Interval   *result;
-
+       
        result = (Interval *) palloc(sizeof(Interval));
 
        if (factor == 0.0)
@@ -2558,27 +2567,26 @@ interval_div(PG_FUNCTION_ARGS)
                                (errcode(ERRCODE_DIVISION_BY_ZERO),
                                 errmsg("division by zero")));
 
-       month_remainder = span->month / factor;
-       day_remainder = span->day / factor;
-       result->month = (int32) month_remainder;
-       result->day = (int32) day_remainder;
-       month_remainder -= result->month;
-       day_remainder -= result->day;
+       result->month = (int32) (span->month / factor);
+       result->day = (int32) (span->day / factor);
 
        /*
-        * Handle any fractional parts the same way as in interval_mul.
+        *      Fractional months full days into days.  See comment in 
+        *      interval_mul().
         */
+       month_remainder_days = (orig_month * (double)DAYS_PER_MONTH) / factor -
+                       result->month * (double)DAYS_PER_MONTH;
+       sec_remainder = (orig_day * (double)SECS_PER_DAY) / factor -
+                       result->day * (double)SECS_PER_DAY +
+                       (month_remainder_days - (int32) month_remainder_days) * SECS_PER_DAY;
 
-       /* fractional months full days into days */
-       month_remainder_days = month_remainder * DAYS_PER_MONTH;
+       /* cascade units down */
        result->day += (int32) month_remainder_days;
-       /* fractional months partial days into time */
-       day_remainder += month_remainder_days - (int32) month_remainder_days;
-
 #ifdef HAVE_INT64_TIMESTAMP
-       result->time = rint(span->time / factor + day_remainder * USECS_PER_DAY);
+       result->time = rint(span->time / factor + sec_remainder * USECS_PER_SEC);
 #else
-       result->time = span->time / factor + day_remainder * SECS_PER_DAY;
+       /* See TSROUND comment in interval_mul(). */
+       result->time = span->time / factor + TSROUND(sec_remainder);
 #endif
 
        PG_RETURN_INTERVAL_P(result);

diff --git a/src/test/regress/expected/interval.out b/src/test/regress/expected/interval.out
index 7a43e90d5dbc34329623b6796b182b3a8506c0c9..0adda4a981d26e5f05fb01183a1b9dab960d361d 100644 (file)
--- a/src/test/regress/expected/interval.out
+++ b/src/test/regress/expected/interval.out
@@ -218,7 +218,7 @@ SELECT '' AS ten, * FROM INTERVAL_TBL;
 select avg(f1) from interval_tbl;
                        avg                       
 -------------------------------------------------
- @ 4 years 1 mon 9 days 28 hours 18 mins 23 secs
+ @ 4 years 1 mon 10 days 4 hours 18 mins 23 secs
 (1 row)
 
 -- test long interval input