TAJO-1265: min(), max() does not handle null properly

[sirpkt]

min() and max() handle null value separately from non-null values.
It computes min() or max() among non-null values first.
If no non-null value exists, they return null value.
Implementation style of min() and max() is also changed that
there are abstract classes of Min and Max common for all the data types
and only type specific methods are implemented for each type.

[hyunsik]

This patch seems to not pass 'mvn clean install'.

[sirpkt]

@hyunsik Sorry for the incomplete patch.
There are many test cases related with max(), min() and I missed the testing for all those cases.
I apologize for stealing your time by faulty patch.
I'll check and update the patch.

[sirpkt]

I updated the patch as followings

• null handling bug fix in getPartialResult() of Min and Max
• getEmptyTuple() in DistinctGroupbySortAggregationExec.java is changed to make Tuple according to the type of aggregation functions: currently, null datum for min and max and 0 for other types.
• Test result is fixed as correct value

mvn clean install passed.

sum() and avg() also need to be changed to handle null differently from non-null values.
I'll make another issue about sum() and avg().

[sirpkt]

I also modified sum() and avg() to handle null properly.

[hyunsik]

I'll review it tonight. Thank you for your contribution.

[hyunsik]

The patch looks good to me. It corrects the wrong implementation. There are some trivial things that I need to check. I'll finish the review by tomorrow.

[hyunsik]

Unused import

[hyunsik]

+1
Thank you for finding these bugs. It seems to correct wrong null handling behavior of aggregation operators.

[hyunsik]

Hi @sirpkt,

I leave some trivial comments. Because they are trivial, you can commit after you remove them.

[sirpkt]

Thank you for the review, @hyunsik.
I removed unused imports.
I checked 'mvn clean install', however, I'll commit after Travis CI build is done without problem.

[sirpkt]

I just committed the patch.