I think I've finally identified the cause of the off-by-one-second

issue in timestamp conversion that we hacked around for so long by
ignoring the seconds field from localtime().  It's simple: you have
to watch out for platform-specific roundoff error when reducing a
possibly-fractional timestamp to integral time_t form.  In particular
we should subtract off the already-determined fractional fsec field.
This should be enough to get an exact answer with int64 timestamps;
with float timestamps, throw in a rint() call just to be sure.

diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 1705441329fbc39ef6f3303befa6665b99c8e9df..d40715b7e44ee148553af53b0a05849d992ac236 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 1.106 2004/05/21 05:08:02 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.107 2004/05/31 18:31:51 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -933,22 +933,18 @@ dt2time(Timestamp jd, int *hour, int *min, int *sec, fsec_t *fsec)
  * local time zone. If out of this range, leave as GMT. - tgl 97/05/27
  */
 int
-timestamp2tm(Timestamp dt, int *tzp, struct pg_tm * tm, fsec_t *fsec, char **tzn)
+timestamp2tm(Timestamp dt, int *tzp, struct pg_tm *tm, fsec_t *fsec, char **tzn)
 {
 #ifdef HAVE_INT64_TIMESTAMP
-   int         date,
-               date0;
+   int         date;
    int64       time;
 #else
-   double      date,
-               date0;
+   double      date;
    double      time;
 #endif
    time_t      utime;
    struct pg_tm  *tx;
 
-   date0 = POSTGRES_EPOCH_JDATE;
-
    /*
     * If HasCTZSet is true then we have a brute force time zone
     * specified. Go ahead and rotate to the local time zone since we will
@@ -983,11 +979,11 @@ timestamp2tm(Timestamp dt, int *tzp, struct pg_tm * tm, fsec_t *fsec, char **tzn
 #endif
 
    /* Julian day routine does not work for negative Julian days */
-   if (date < -date0)
+   if (date < -POSTGRES_EPOCH_JDATE)
        return -1;
 
    /* add offset to go from J2000 back to standard Julian date */
-   date += date0;
+   date += POSTGRES_EPOCH_JDATE;
 
    j2date((int) date, &tm->tm_year, &tm->tm_mon, &tm->tm_mday);
    dt2time(time, &tm->tm_hour, &tm->tm_min, &tm->tm_sec, fsec);
@@ -1014,11 +1010,19 @@ timestamp2tm(Timestamp dt, int *tzp, struct pg_tm * tm, fsec_t *fsec, char **tzn
         */
        else if (IS_VALID_UTIME(tm->tm_year, tm->tm_mon, tm->tm_mday))
        {
+           /*
+            * Convert to integer, avoiding platform-specific
+            * roundoff-in-wrong-direction errors, and adjust to
+            * Unix epoch.  Note we have to do this in one step
+            * because the intermediate result before adjustment
+            * won't necessarily fit in an int32.
+            */
 #ifdef HAVE_INT64_TIMESTAMP
-           utime = ((dt / INT64CONST(1000000))
-                    + ((date0 - UNIX_EPOCH_JDATE) * INT64CONST(86400)));
+           utime = (dt - *fsec) / INT64CONST(1000000) +
+               (POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * 86400;
 #else
-           utime = (dt + ((date0 - UNIX_EPOCH_JDATE) * 86400));
+           utime = rint(dt - *fsec +
+                        (POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * 86400);
 #endif
 
            tx = pg_localtime(&utime);