[Impala-CR](cdh5-2.6.0_5.8.0) IMPALA-3163: Fix Decimal to Timestamp casting

["Taras Bobrovytsky (Code Review)" <ge...@cloudera.org>]

Taras Bobrovytsky has uploaded a new change for review.

  http://gerrit.cloudera.org:8080/3154

Change subject: IMPALA-3163: Fix Decimal to Timestamp casting
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IMPALA-3163: Fix Decimal to Timestamp casting

Before this patch, we would first convert the Decimal to Double, then
Double to Timestamp. This resulted in imprecise results.

I ran a benchmark where we read decimal values from a large parquet
table and cast them to timestamp. The new correct implementation is
slightly slower than the old one (96 seconds vs 70 seconds). This issue
can probably be fixed with codegen.

Change-Id: Iabeea9f4ab4880b2f814408add63c77916e2dba9
---
M be/src/exprs/decimal-operators-ir.cc
M be/src/exprs/expr-test.cc
2 files changed, 40 insertions(+), 5 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/Impala refs/changes/54/3154/1
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Gerrit-MessageType: newchange
Gerrit-Change-Id: Iabeea9f4ab4880b2f814408add63c77916e2dba9
Gerrit-PatchSet: 1
Gerrit-Project: Impala
Gerrit-Branch: cdh5-2.6.0_5.8.0
Gerrit-Owner: Taras Bobrovytsky <tb...@cloudera.com>