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https://issues.apache.org/jira/browse/FLINK-9430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16490368#comment-16490368
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Timo Walther commented on FLINK-9430:
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My concern was that JSON is also parsing from String to some data type. Why not
moving the parsing to Flink using CAST. But I agree that this is not always
easy esp. with arrays and maps. So lets allow casting of Object. Btw would you
be interested in helping with JSON function support in Flink? There is also
some interest in Calcite for this (CALCITE-2266). It would be great to have a
end-to-end JSON story in Flink SQL.
> Support Casting of Object to Primitive types for Flink SQL UDF
> --------------------------------------------------------------
>
> Key: FLINK-9430
> URL: https://issues.apache.org/jira/browse/FLINK-9430
> Project: Flink
> Issue Type: New Feature
> Components: Table API & SQL
> Reporter: Shuyi Chen
> Assignee: Shuyi Chen
> Priority: Major
>
> We want to add a SQL UDF to access specific element in a JSON string using
> JSON path. However, the JSON element can be of different types, e.g. Int,
> Float, Double, String, Boolean and etc.. Since return type is not part of the
> method signature, we can not use overload. So we will end up writing a UDF
> for each type, e.g. GetFloatFromJSON, GetIntFromJSON and etc., which has a
> lot of duplication.
> One way to unify all these UDF functions is to implement one UDF and return
> java.lang.Object, and in the SQL statement, use CAST AS to cast the returned
> Object into the correct type. Below is an example:
>
> {code:java}
> object JsonPathUDF extends ScalarFunction {
> def eval(jsonStr: String, path: String): Object = {
> JSONParser.parse(jsonStr).read(path)
> }
> }{code}
> {code:java}
> SELECT CAST(jsonpath(json, "$.store.book.title") AS VARCHAR(32)) as
> bookTitle FROM table1{code}
> The current Flink SQL cast implementation does not support casting from
> GenericTypeInfo<java.lang.Object> to another type, I have already got a local
> branch to fix this. Please comment if there are alternatives to the problem
> above.
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