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