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https://issues.apache.org/jira/browse/HIVE-5878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13843348#comment-13843348
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Xuefu Zhang commented on HIVE-5878:
-----------------------------------
Backward compatibility issue is a valid concern. However, before we decide to
enforce or break that, we need to ask ourselves whether the new behavior is the
right thing to do and how bad the old behavior is. We had questionings about
why we need to keep backward compatible to a bug.
As an example, Hive returned double for int/int previously, but now returns
decimal. This is a bigger backward compatibility problem, but obviously it's a
right thing to do. The issue in the discussion seems following the same line.
Also, I think hive is still in its early days, and it doesn't even have 1.0
release yet. Thus, personally, BC issue is less a concern than doing right
thing.
> 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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