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I think it has to do with the basic concepts of SQL and Timestamp datatype.
The Timestamp datatype stores fractional seconds of time. So, when you perform a "group by" on a Timestamp column the probability of 2 or more rows having the exact same fractional seconds is almost zero.
As a consequence there is practically no grouping / aggregation that takes place and the number of rows returned are almost the same as without the group by clause.
Food for thought?
It really depends on the granularity that you need from the timestamp to make it meaningful. The key word is aggregation. So you should ask what level of aggregation do I need?
You can reduce the granularity from the fractional seconds to some lower level of granularity like seconds, minute, hours or even date (equivalent of trunc(date_column) using CAST or Truncating or to_date(to_char(...)) with appropriate format mask.
It just depends on the application and data.
E.g. for the LHC at CERN chasing the Higgins, timestamp is just not fine grained enough (you need 10 exp -23 or lower I guess !)
E.g. if you look at seismic data for oil exploration a hell of a lot happens in 5th and 6th decimal places in the timestamp.
But if you are looking at data logged by a normal SCADA system then maybe a second is detailed enough for the purpose.
In normal business application we are better off "rounding" the timestamp to some meaningful level for aggregation/ reporting.
Not sure how you are trying it. I am able to make an IR with Group By on Timestamp with Min and Max functions for other columns.
Here is how I did it, just for you to compare the steps.
a. Create an IR with a simple query with 2 columns, Reference_No (Varchar2) and Date_Posted (Timestamp). No group by or aggregation fucntions
b. At run time Actions > Format > Group By
c. Selected the Timestamp column in Group By Column 1
d. Added Min > Reference_no > Min Ref in the first row of the first table
e. Added Max > Reference_no > Max Ref in the second row
f. Clicked Apply
It works !
Sorry for my poor english what I mean is the TIME datatypes are not available as columns for functions like min and max.
the anser on metalink
As you have noticed date, timestamp do not allow to use functions like min, max, etc.. in iteractive report.
Currently those functions are only supported for columns of type number.
The problem has been addressed to development as bug:
Bug 10247814: INTERACTIVE REPORT AGGREGATE AND GROUP BY SUPPORTS AGGREGATE ONLY ON NUMBERS
As far I can see it will be addressed in the next release (apex 4.2). Unfortunately we do not have a timeframe for the next release.
For the time being, please monitor the bug via My Oracle Support.