Rename `expr::window_function::WindowFunction` to `WindowFunctionDefintion` for consistency

[alamb]

Is your feature request related to a problem or challenge?

Part of #8045 we are working
to consolidate the function implementation in DataFusion to make it easier to
manage DataFusion's built in list of functions as well as ensure user defined
and built in functions have the same feature sets.

I think it is also important to keep the Expr representation consistent for
Aggregate and Window functions to make DataFusion easier to work with, as well
as to permit eventually applying the same unification of built in and user
defined functions to aggregates and window functions.

Describe the solution you'd like

I would like to rename the WindowFunction in to https://github.com/apache/arrow-datafusion/blob/4c914ea1e5d3dc61f3552adcffb8356c90d73bac/datafusion/expr/src/window_function.rs#L35-L44 to WindowFunctionDefinition so we didn't have so many things named WindowFunction and to mirror the structure of ScalarFunction

Edited: I would like to unify Expr::WindowFunction and Expr::WindowUDF following the pattern in #8258 @2010YOUY01:

Describe alternatives you've considered

No response

Additional context

I think this is a pretty good first issue because we have an existing pattern in #8258

[haohuaijin]

it seems like we already unified WindowFunction in
https://github.com/apache/arrow-datafusion/blob/4c914ea1e5d3dc61f3552adcffb8356c90d73bac/datafusion/expr/src/expr.rs#L515-L526
and
https://github.com/apache/arrow-datafusion/blob/4c914ea1e5d3dc61f3552adcffb8356c90d73bac/datafusion/expr/src/window_function.rs#L35-L44

[alamb]

it seems like we already unified WindowFunction in

That is an excellent point. Thank you for the research @haohuaijin

Maybe we could rename https://github.com/apache/arrow-datafusion/blob/4c914ea1e5d3dc61f3552adcffb8356c90d73bac/datafusion/expr/src/window_function.rs#L35-L44 to WindowFunctionDefinition so we didn't have so many things named WindowFunction and to mirror the structure of ScalarFunctions

[edmondop]

@haohuaijin are you already working on it or can I pick it up?

[haohuaijin]

@edmondop I haven't started working yet, feel free to pick it up.

[edmondop]

@alamb I am probably losing some pre-requisite, the Expr doesn't contain a WindowUDF variant but only a WindowFunction variant https://github.com/apache/arrow-datafusion/blob/main/datafusion/expr/src/expr.rs#L156 there is an AggregateUDF though...

[alamb]

@alamb I am probably losing some pre-requisite, the Expr doesn't contain a WindowUDF variant but only a WindowFunction variant https://github.com/apache/arrow-datafusion/blob/main/datafusion/expr/src/expr.rs#L156 there is an AggregateUDF though...

You are right that the ticket description was incorrect, as @haohuaijin noted in #8347 (comment)

I have updated it to say "rename WindowFunctiontoWindowFunctionDefinition`" -- sorry for the confusion

I think AggregateUDF is covered by #8346