[IMPALA-4626] Different results from power() with cast decimal expression vs. the equivalent literal value as base parameter

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Duplicate
• Affects Version/s: Impala 2.3.0
• Component/s: Backend
• Environment: CDH 5.5.2 (Isilon)
• Epic Link: Numeric Datatype Conversions
• Assignee: Unassigned
• Reporter: Curtis Howard
• Created: 08/Dec/16 02:04
• Updated: 13/Mar/17 18:23
• Resolved: 03/Mar/17 01:07

Description

The script below highlights the issue we are seeing. If the result of CAST(base as DECIMAL) is provided to power() as a literal the result is ~0.0001% different as compared to when substituting the CAST expression as the base parameter.

Running the same test in Hive 1.1.0 the two alternatives produce the same result (to the 16th decimal place).

Is this expected behavior in Impala?

{{
CREATE TABLE test_power (base DECIMAL(29,5), exponent DECIMAL(11,0));
INSERT INTO test_power VALUES(0.03740,43);

SELECT
CAST(base as DECIMAL(38,16)) AS cast_base_result,
POWER(0.0374000000000000, -1 * exponent) AS power_result_using_literal_base,
POWER(CAST(base as DECIMAL(38,16)), -1 * exponent) AS power_result_using_decimal_expr_base
FROM test_power
}}

------------------------------------------- output ---------------------------------------
cast_base_result=0.0374000000000000
power_result_using_literal_base=2.3255254965315044e+61
power_result_using_decimal_expr_base=2.3255282221097883e+61

Issue Links

• duplicates: IMPALA-5014 DECIMAL V2 round when casting to/from DECIMAL, part 2