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https://issues.apache.org/jira/browse/AVRO-2899?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17422590#comment-17422590
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Ryan Skraba commented on AVRO-2899:
-----------------------------------
I linked the equivalent JIRA about reading the union without the extra
indirection of the type! This is such a frequent question, and not very well
documented.
Changing the JSON encoding is a specification change that would need some extra
thought. I'm definitely not against making a change that makes the JSON format
"less surprising", but we should prioritize users that depend on
interoperability between languages and versions.
In practice, I try to avoid the JSON representation of Avro data and this is
one of the reasons why.
> JsonEncoder writes type information for not-null union
> ------------------------------------------------------
>
> Key: AVRO-2899
> URL: https://issues.apache.org/jira/browse/AVRO-2899
> Project: Apache Avro
> Issue Type: Improvement
> Components: java
> Affects Versions: 1.9.2
> Reporter: Werner Daehn
> Priority: Major
>
> _Summary: A union of [null,string] should output the value as such when using
> the JsonEncoder. To accomplish that, a single line needs to be changed in the
> JsonEncoder.java. I don't believe there are side effects but not sure -
> looking for validation._
>
> When the schema looks like
> {{name: "text", type: ["null",\{"type":"string"}] }}
> the JsonEncoder creates a Json object explicitly stating the type. So the
> created json is
> {{text: \{ "string": "Hello World" }}}
> instead of
> {{text: "Hello World"}}
> I have searched for this issue and people complain frequently but no real
> resolution.
> While I understand why this is done, the JsonDecoder needs to know what type
> to add in case of a union, it does not make sense in a not-null union where
> there is either a string value or not.
>
> I would argue, in the simple case where there is a union of two elements and
> the first is the NULL symbol, the extra text can be omitted. Hence I have
> changed the writeIndex line in the JsonEncoder from
> [https://github.com/apache/avro/blob/c903aa6d6fc42d3c347f95d469a8364ea44165e8/lang/java/avro/src/main/java/org/apache/avro/io/JsonEncoder.java#L297]
> {{if (symbol != Symbol.NULL) {}}
> to
> {{if (symbol != Symbol.NULL && (top.symbols.length > 2 || top.getSymbol(0) !=
> Symbol.NULL)) {}}
>
> It should not have a side effect on the JsonDecoder either, as in case of
> schema evolution - making a field null-able - this must be resolved anyhow. I
> am not sure about that however.
>
>
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