[jira] [Created] (FLINK-12711) Separate function implementation and definition

[Shang Yuanchun (Jira)]

Timo Walther created FLINK-12711:
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             Summary: Separate function implementation and definition
                 Key: FLINK-12711
                 URL: https://issues.apache.org/jira/browse/FLINK-12711
             Project: Flink
          Issue Type: Sub-task
          Components: Table SQL / API
            Reporter: Timo Walther
            Assignee: Timo Walther


This issue continues the work that was started in FLINK-11449. It distinguishes between function with implementation (UDFs) and function with no implementation.

The rough design looks as follows:

{noformat}
1. `interface FunctionDefinition`
   --> general interface for describing a function
   --> goal: separation of runtime and planning/optimization
   --> long-term methods: `getKind()` (aggregate, scalar, table), `getTypeInference()`
2. `interface UserDefinedFunctionDefinition extends FunctionDefinition`
   --> interface for describing a function with implementation
   --> methods: `createImplementation(): UserDefinedFunction`
   --> default: getTypeInference() = Util.DEFAULT_INFERENCE // future work
3. `class BuiltInFunctionDefinition implements FunctionDefinition`
   --> class for describing a function where the planner provides an implementation
   --> methods: `getName(): String`
4. Add `getKind` to `AggregateFunction`, `ScalarFunction`, `TableFunction`
{noformat}



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