[FLINK-9430] Support Casting of Object to Primitive types for Flink SQL UDF

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Component/s: Table SQL / API
• Labels: pull-request-available
• Assignee: Unassigned
• Reporter: Shuyi Chen
• Created: 24/May/18 06:27
• Updated: 21/Jul/20 07:52
• Resolved: 21/Jul/20 07:51

Description

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:

 

object JsonPathUDF extends ScalarFunction {
 def eval(jsonStr: String, path: String): Object = {
   JSONParser.parse(jsonStr).read(path)
 }
}

 SELECT CAST(jsonpath(json, "$.store.book.title") AS VARCHAR(32)) as bookTitle FROM table1

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.

Issue Links

• is related to: FLINK-9484 Improve generic type inference for User-Defined Functions
• links to: GitHub Pull Request #6100
• links to: GitHub Pull Request #6100