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https://issues.apache.org/jira/browse/HIVE-5878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13843882#comment-13843882
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Xuefu Zhang commented on HIVE-5878:
-----------------------------------
For information, the follow is the text from SQL-92[1] standard w.r.t AVG
function:
{quote}
c) If AVG is specified and DT is exact numeric, then the data
type of the result is exact numeric with implementation-
defined precision not less than the precision of DT and
implementation-defined scale not less than the scale of DT.
{quote}
Clearly, currently Hive deviates from this. Both MySQL and SQL server are in
line with this.
[1] http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
> Hive standard avg UDAF returns double as the return type for some exact input
> types
> -----------------------------------------------------------------------------------
>
> Key: HIVE-5878
> URL: https://issues.apache.org/jira/browse/HIVE-5878
> Project: Hive
> Issue Type: Bug
> Components: Types, UDF
> Affects Versions: 0.12.0
> Reporter: Xuefu Zhang
> Assignee: Xuefu Zhang
> Attachments: HIVE-5878.1.patch, HIVE-5878.patch
>
>
> For standard, no-partial avg result, hive currently returns double as the
> result type.
> {code}
> hive> desc test;
> OK
> d int None
> Time taken: 0.051 seconds, Fetched: 1 row(s)
> hive> explain select avg(`d`) from test;
> ...
> Reduce Operator Tree:
> Group By Operator
> aggregations:
> expr: avg(VALUE._col0)
> bucketGroup: false
> mode: mergepartial
> outputColumnNames: _col0
> Select Operator
> expressions:
> expr: _col0
> type: double
> {code}
> However, exact types including integers and decimal should yield exact type.
> Here is what MySQL does:
> {code}
> mysql> desc test;
> +-------+--------------+------+-----+---------+-------+
> | Field | Type | Null | Key | Default | Extra |
> +-------+--------------+------+-----+---------+-------+
> | i | int(11) | YES | | NULL | |
> | b | tinyint(1) | YES | | NULL | |
> | d | double | YES | | NULL | |
> | s | varchar(5) | YES | | NULL | |
> | dd | decimal(5,2) | YES | | NULL | |
> +-------+--------------+------+-----+---------+-------+
> mysql> create table test62 as select avg(i) from test;
> mysql> desc test62;
> +-------+---------------+------+-----+---------+-------+
> | Field | Type | Null | Key | Default | Extra |
> +-------+---------------+------+-----+---------+-------+
> | avg(i) | decimal(14,4) | YES | | NULL | |
> +-------+---------------+------+-----+---------+-------+
> 1 row in set (0.00 sec)
> {code}
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