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From "Michael Ho (Code Review)" <ger...@cloudera.org>
Subject [Impala-ASF-CR] IMPALA-2020: Add rounding for decimal casts
Date Wed, 15 Feb 2017 22:57:49 GMT
Michael Ho has posted comments on this change.

Change subject: IMPALA-2020: Add rounding for decimal casts
......................................................................


Patch Set 10:

(9 comments)

http://gerrit.cloudera.org:8080/#/c/5951/10//COMMIT_MSG
Commit Message:

PS10, Line 10: Still writing tests,
             : but this is ready for human eyes to look at.
Not needed in the commit message anymore, right ?


http://gerrit.cloudera.org:8080/#/c/5951/5/be/src/exprs/literal.cc
File be/src/exprs/literal.cc:

Line 185:         value_.decimal4_val = Decimal4Value::FromDouble(type, v, true, &overflow);
> This functionality (constructing Literals given a ColumnType) is only used 
Shouldn't this still be dependent on the behavior we want to test ? We test both the v1 vs
v2 behavior in the BE test. May consider taking an extra argument (bool round) ?


http://gerrit.cloudera.org:8080/#/c/5951/10/be/src/runtime/decimal-value.h
File be/src/runtime/decimal-value.h:

PS10, Line 237: inline typename RESULT_T::underlying_type_t ToInt(int scale, bool round, bool*
overflow) const;
long line


http://gerrit.cloudera.org:8080/#/c/5951/10/be/src/runtime/decimal-value.inline.h
File be/src/runtime/decimal-value.inline.h:

PS10, Line 38:   if (round) {
             :     // Decimal V2 behavior
             : 
             :     // Multiply the double by the scale.
             :     //
             :     // TODO: this could be made more precise by doing the computation in
             :     // 64 bit with 128 bit immediate result instead of doing an additional
             :     // multiply in 53-bit double precision floating point
             : 
             :     d *= DecimalUtil::GetScaleMultiplier<double>(scale);
             :     d = std::round(d);
             :     const T max_value = DecimalUtil::GetScaleMultiplier<T>(precision);
             :     if (abs(d) >= max_value) {
             :       *overflow = true;
             :       return DecimalValue();
             :     }
             : 
             :     // Return the rounded integer part.
             :     return DecimalValue(static_cast<T>(d));
             :   } else {
             :     // TODO: IMPALA-4924: remove DECIMAL V1 code
             : 
             :     // Check overflow.
             :     const T max_value = DecimalUtil::GetScaleMultiplier<T>(precision
- scale);
             :     if (abs(d) >= max_value) {
             :       *overflow = true;
             :       return DecimalValue();
             :     }
             : 
             :     // Multiply the double by the scale.
             :     d *= DecimalUtil::GetScaleMultiplier<double>(scale);
             : 
             :     // Truncate and just take the integer part.
             :     return DecimalValue(static_cast<T>(d));
             :   }
Appears to me that we should be able to share the code for the most part:

d *= DecimalUtil::GetScaleMultiplier<double>(scale);
if (round) d = std::round(d);
const T max_value = DecimalUtil::GetScaleMultiplier<T>(precison);
if (abs(d) >= max_value) {
   ...
}

return DecimalValue(static_cast<T>(d));


PS10, Line 108: typename RESULT_T::underlying_type_t
May help to add a comment that underlying_type_t is defined only for the *IntType in udf.h


Line 113:   if (divisor == 1) {
> what's this for? is it an optimization for scale==0 case?
If it's for optimization, constant propagation in codegen would have reaped most of the benefit
as this is inlined into cross-compiled code.


PS10, Line 117: const T remainder = v % divisor;
Do we not need to compute this if round is false ?


PS10, Line 119: DCHECK(
DCHECK_EQ


PS10, Line 127: typename RESULT_T::underlying_type_t
May be consider doing a typedef typename RESULT_T::underlying_type_t result_type; inside here
for ease of reading.


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Gerrit-MessageType: comment
Gerrit-Change-Id: I2daf186b4770a022f9cb349d512067a1dd624810
Gerrit-PatchSet: 10
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Zach Amsden <zamsden@cloudera.com>
Gerrit-Reviewer: Dan Hecht <dhecht@cloudera.com>
Gerrit-Reviewer: Michael Ho
Gerrit-Reviewer: Michael Ho <kwho@cloudera.com>
Gerrit-Reviewer: Zach Amsden <zamsden@cloudera.com>
Gerrit-HasComments: Yes

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