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https://issues.apache.org/jira/browse/IMPALA-4939?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Taras Bobrovytsky resolved IMPALA-4939.
---------------------------------------
Resolution: Fixed
{code}
commit 6259641077250dcd3360e2e6c2bf9023b201d858
Author: Taras Bobrovytsky <[email protected]>
Date: Thu Jul 6 10:43:34 2017 -0700
IMPALA-4939, IMPALA-4940: Decimal V2 multiplication
Implement the new DECIMAL return type rules for multiply expressions,
active when query option DECIMAL_V2=1. The algorithm for determining
the type of the result of multiplication is described in the JIRA.
{code}
> DECIMAL multiply overflows early
> --------------------------------
>
> Key: IMPALA-4939
> URL: https://issues.apache.org/jira/browse/IMPALA-4939
> Project: IMPALA
> Issue Type: Bug
> Components: Backend
> Affects Versions: Impala 2.9.0
> Reporter: Dan Hecht
> Assignee: Taras Bobrovytsky
>
> Decimal multiplication indicates overflow before you've actually overflowed
> the result type. For example:
> {code}
> [dhecht-desktop.ca.cloudera.com:21000] > select typeof(cast(.1 as
> decimal(20,20)) * cast(1 as decimal(20,19)));
> Query: select typeof(cast(.1 as decimal(20,20)) * cast(1 as decimal(20,19)))
> Query submitted at: 2017-02-15 14:08:11 (Coordinator:
> http://dhecht-desktop.ca.cloudera.com:25000)
> Query progress can be monitored at:
> http://dhecht-desktop.ca.cloudera.com:25000/query_plan?query_id=ac4e45202a0850c4:c757c44600000000
> +-----------------------------------------------------------------+
> | typeof(cast(0.1 as decimal(20,20)) * cast(1 as decimal(20,19))) |
> +-----------------------------------------------------------------+
> | DECIMAL(38,38) |
> +-----------------------------------------------------------------+
> [dhecht-desktop.ca.cloudera.com:21000] > select cast(.1 as decimal(20,20)) *
> cast(1 as decimal(20,19));
> Query: select cast(.1 as decimal(20,20)) * cast(1 as decimal(20,19))
> Query submitted at: 2017-02-15 14:07:58 (Coordinator:
> http://dhecht-desktop.ca.cloudera.com:25000)
> Query progress can be monitored at:
> http://dhecht-desktop.ca.cloudera.com:25000/query_plan?query_id=c94daa3dc258be9c:559353e800000000
> +---------------------------------------------------------+
> | cast(0.1 as decimal(20,20)) * cast(1 as decimal(20,19)) |
> +---------------------------------------------------------+
> | NULL |
> +---------------------------------------------------------+
> WARNINGS: UDF WARNING: Expression overflowed, returning NULL
> [dhecht-desktop.ca.cloudera.com:21000] > select cast(.1 as decimal(20,20)) *
> cast(1 as decimal(19,18));
> Query: select cast(.1 as decimal(20,20)) * cast(1 as decimal(19,18))
> Query submitted at: 2017-02-15 14:12:48 (Coordinator:
> http://dhecht-desktop.ca.cloudera.com:25000)
> Query progress can be monitored at:
> http://dhecht-desktop.ca.cloudera.com:25000/query_plan?query_id=8d4a0189a544e194:12148c3100000000
> +---------------------------------------------------------+
> | cast(0.1 as decimal(20,20)) * cast(1 as decimal(19,18)) |
> +---------------------------------------------------------+
> | 0.10000000000000000000000000000000000000 |
> +---------------------------------------------------------+
> Fetched 1 row(s) in 0.01s
> {code}
> I.e. {{0.1 * 1}} fits in a DECIMAL(38,38), but our implementation of decimal
> multiply can overflow when S1+S2 > 38 even if the result would fit the result
> type.
> The problem is that when starting with Decimal16Values, we do a 128 * 128 =>
> 128-bit multiply, rather than 128 * 128 => 256 bit multiply. We check the
> intermediate result which has full (S1+S2) scale against
> MAX_UNSCALED_DECIMAL16, even though we later have code to scale down the
> result as needed to fit the result type.
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