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From "Kim Haase (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-6127) Tell users to make sure that the state of a user-defined aggregate needs to be serializable
Date Tue, 10 Sep 2013 21:13:51 GMT

    [ https://issues.apache.org/jira/browse/DERBY-6127?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13763522#comment-13763522
] 

Kim Haase commented on DERBY-6127:
----------------------------------

Here's another question, about this sentence:

"A user-defined aggregate may end up being serialized to disk when performing grouped aggregation
over a large number of groups."

What is it that is performing the grouped aggregation? Does the aggregate itself do this,
as the syntax suggests? Or is it a query that does it, or something else?
                
> Tell users to make sure that the state of a user-defined aggregate needs to be serializable
> -------------------------------------------------------------------------------------------
>
>                 Key: DERBY-6127
>                 URL: https://issues.apache.org/jira/browse/DERBY-6127
>             Project: Derby
>          Issue Type: Improvement
>          Components: Documentation
>    Affects Versions: 10.10.1.1
>            Reporter: Rick Hillegas
>            Assignee: Kim Haase
>
> The Reference Manual section on "CREATE DERBY AGGREGATE statement" should tell users
to make sure that all of the state of their user-defined aggregate is really serializable.
This is because the Aggregator interface extends java.io.Serializable. A user-defined aggregate
may end up being serialized to disk when performing grouped aggregation over a large number
of groups. That is, intermediate results may be serialized to disk for a query like the following.
The serialization will fail if the user-defined aggregate contains non-serializable fields:
>   select a, myAggregate( b ) from myTable group by a

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