[jira] [Commented] (AVRO-1803) nullable fields and it's default value

["Ryan Blue (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AVRO-1803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15165682#comment-15165682 ] 

Ryan Blue commented on AVRO-1803:
---------------------------------

I think the wording is somewhat confusing, but actually correct. When it says "the default value of such unions is *typically* null" it means that users commonly add a null default by explicitly adding it. If you don't add a default, the readers expect a value to be present.

The default is only used when there is no value in the encoded bytes, but the reader expects one. The reader either gets the default that was set in the read schema, or will get an exception that there is no default value and the data file was missing the value.

> nullable fields and it's default value
> --------------------------------------
>
>                 Key: AVRO-1803
>                 URL: https://issues.apache.org/jira/browse/AVRO-1803
>             Project: Avro
>          Issue Type: Bug
>          Components: build
>    Affects Versions: 1.7.7, 1.8.0
>            Reporter: Marcel Silberhorn
>            Priority: Minor
>
> as described in
> https://avro.apache.org/docs/current/spec.html
> {quote}h6. Unions
> Unions, as mentioned above, are represented using JSON arrays. For example, {{\["null", "string"\]}} declares a schema which may be either a null or string.
> (Note that when a default value is specified for a record field whose type is a union, the type of the default value must match the +first+ element of the union. Thus, for unions containing "null", the "null" is usually listed first, since the default value of such unions is typically null.){quote}
> and the given example for Handshake with
> {code}
> {"name": "clientProtocol", "type": ["null", "string"]},
> {code}
> or even the "tweet.hashtags" example from http://stackoverflow.com/questions/31864450
> as seen in http://stackoverflow.com/questions/9417732 you have to explicit define the {{"default": null}} for the "nullable union field", else you get
> {{Field clientProtocol type:UNION pos:0 not set and has no default value}}
> so either there is a bug in the documentation or in the code ,)
> If its "just" a documentation error, please decrease this bug to a wish: add some compiler warnings when using union without default value



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