Support alias function, Java UDF, Java UDAF for Nereids.
Implementation:
UDFs(alias function, Java UD(A)F) are saved in database object, we get it by FunctionDesc, which requires function name and arg types. So firstly we bind expressions of its children so that we can get the return type of args. Then we get the best selection.
Secondly:
For alias function:
The original function of the alias function is represented as original planner-style function, it's too hard to translate it to nereids-style expression hence we transfer it to the corresponding sql and parse it. Now we get the nereids-style function, and try to bind the function.
the bound function will also change the type by add cast node of its children to its expecting input types, so that if we travel a bound function more than one times, the cast node will be different. To solve the problem, we add a flag isAnalyzedFunction. it's set false by default and will be set true when return from the visitor function. If the flag is true, it will return immediately in visitor function.
Now we can ensure that the bound functions in children will be the same though we travel it more than one time. we can replace the alias function to its original function and bind the unbound functions.
For JavaUDF and JavaUDAF
JavaUDF and JavaUDAF can be recognized as a catalog function and hard to be entirely translated to Nereids-style function, we create a nereids expression object JavaUdf and JavaUdaf to wrap it.
All in all, now Nereids support UDFs and nesting them.
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