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https://issues.apache.org/jira/browse/FLINK-9430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16494761#comment-16494761
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Shuyi Chen commented on FLINK-9430:
-----------------------------------
Hi [~twalthr], I've attached the PR. Having an end-to-end JSON story in Flink
SQL will be great. I'll create a separate ticket for it, we will need to first
get SQL JSON grammar ready in Calcite.
> 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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