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https://issues.apache.org/jira/browse/CASSANDRA-10310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14847365#comment-14847365
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Sylvain Lebresne commented on CASSANDRA-10310:
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Just as a fyi, I actually don't think we can use type casting for this, we
might have to rely on conversion functions for that (though there is
potentially other options). I don't really have time to go into the details of
why today however, so I'll update with more details next week or something.
With that said, I generally agree with the idea of allowing as much as is
reasonable, but I also think that the exact details on what is reasonable is
the least important part of this ticket, and we should actually probably start
with something conservative, since adding new conversion/casting we've
forgotten is easy, it's removing some that turns out to not be such a good idea
that is hard.
> Support type casting in selection clause
> ----------------------------------------
>
> Key: CASSANDRA-10310
> URL: https://issues.apache.org/jira/browse/CASSANDRA-10310
> Project: Cassandra
> Issue Type: Bug
> Reporter: Jon Haddad
> Assignee: Benjamin Lerer
> Labels: patch
> Attachments: cassandra-2.2-10310.txt, cassandra-3.0-10310.txt
>
>
> When selecting an avg() of int values, the type of the avg value returned is
> an int as well, meaning it's rounded off to an incorrect answer. This is
> both incorrect and inconsistent with other databases.
> Example:
> {quote}
> cqlsh:test> select * from monkey where id = 1;
> id | i | v
> ----+---+---
> 1 | 1 | 1
> 1 | 2 | 1
> 1 | 3 | 2
> (3 rows)
> cqlsh:test> select avg(v) from monkey where id = 1;
> system.avg(v)
> ---------------
> 1
> (1 rows)
> {quote}
> I tried avg() with MySQL, here's the result:
> {quote}
> mysql> create table blah ( id int primary key, v int );
> Query OK, 0 rows affected (0.15 sec)
> mysql> insert into blah set id = 1, v = 1;
> Query OK, 1 row affected (0.02 sec)
> mysql> insert into blah set id = 1, v = 1;
> ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
> mysql> insert into blah set id = 2, v = 1;
> Query OK, 1 row affected (0.01 sec)
> mysql> insert into blah set id = 3, v = 2;
> Query OK, 1 row affected (0.01 sec)
> mysql> select avg(v) from blah;
> +--------+
> | avg(v) |
> +--------+
> | 1.3333 |
> +--------+
> 1 row in set (0.00 sec)
> {quote}
> I created a new table using the above query. The result:
> {quote}
> mysql> create table foo as select avg(v) as a from blah;
> Query OK, 1 row affected, 1 warning (0.04 sec)
> Records: 1 Duplicates: 0 Warnings: 1
> mysql> desc foo;
> +-------+---------------+------+-----+---------+-------+
> | Field | Type | Null | Key | Default | Extra |
> +-------+---------------+------+-----+---------+-------+
> | a | decimal(14,4) | YES | | NULL | |
> +-------+---------------+------+-----+---------+-------+
> 1 row in set (0.01 sec)
> {quote}
> It works the same way in postgres, and to my knowledge, every RDBMs.
> Broken in 2.2, 3.0.
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