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https://issues.apache.org/jira/browse/CASSANDRA-10310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14743444#comment-14743444
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Jon Haddad commented on CASSANDRA-10310:
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SQL server and Oracle allow you to cast a column and get whatever value you
want. This decision is a little odd to me, since we're effectively saying "you
choose an int for your column so you don't get precision", and don't offer a
way around it. I can't think of a case where you'd prefer to have a less
precise result from avg().
> avg() on int give incorrect results (returning int instead of decimal or
> float)
> -------------------------------------------------------------------------------
>
> Key: CASSANDRA-10310
> URL: https://issues.apache.org/jira/browse/CASSANDRA-10310
> Project: Cassandra
> Issue Type: Bug
> Reporter: Jon Haddad
> 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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