Add comment documenting actual failure case of using

interval_justify_hours in timestamp subtraction.  TODO already has text
description.

diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index b4a518a1da45320951ad0c1c2ca486d60edaaef1..43956597e31b9d362261e5ff008b2e5b00b0be86 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 1.156 2005/10/25 17:13:07 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/utils/adt/timestamp.c,v 1.157 2005/10/27 02:45:22 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1943,8 +1943,31 @@ timestamp_mi(PG_FUNCTION_ARGS)
        result->month = 0;
        result->day = 0;
 
-       /* this is wrong, but removing it breaks a lot of regression tests */
-       result = DatumGetIntervalP(DirectFunctionCall1(interval_justify_hours,
+       /*
+        *      This is wrong, but removing it breaks a lot of regression tests.
+        *      For example:
+        *
+        *      test=> SET timezone = 'EST5EDT';
+        *      test=> SELECT
+        *      test-> ('2005-10-30 13:22:00-05'::timestamptz -
+        *      test(>  '2005-10-29 13:22:00-04'::timestamptz);
+        *      ?column?
+        *      ----------------
+        *       1 day 01:00:00
+        *       (1 row)
+        *
+        *      so adding that to the first timestamp gets:
+        *
+        *       test=> SELECT
+        *       test-> ('2005-10-29 13:22:00-04'::timestamptz +
+        *       test(> ('2005-10-30 13:22:00-05'::timestamptz -
+        *       test(>  '2005-10-29 13:22:00-04'::timestamptz)) at time zone 'EST';
+        *          timezone
+        *      --------------------
+        *      2005-10-30 14:22:00
+        *      (1 row)
+        */
+       result = DatumGetIntervalP(DirectFunctionCall1(interval_justify_hours,
                                                                                                 IntervalPGetDatum(result)));
 
        PG_RETURN_INTERVAL_P(result);