ARROW-16211: [C++][Python] Unregister compute functions

[vibhatha]

This PR includes a modification to the function registry usage in existing UDF implementation. This allows users to create functions in a given function registry by leveraging the nested function registry approach introduced before. This provides the ability to isolate a group of functions for a defined registry. When functions needs to be removed, deleting the registry including those functions would remove the functions too.

This is not exactly unregistering functions, but a different approach which serves the same purpose.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-16211

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[jorisvandenbossche]

How does Arrow (c++) decide which function registry to look into? And in which order in case there would be conflicting names?

Checking the code, it seems that pc.call_function(..) also only looks in the default, global function registry. So in practice, how useful is it that you can register a UDF in another registry? How does this then actually get used? (how do you call it manually with pc.call_function(..), how does it get resolved if you reference this kernel in a projection or query?)

[vibhatha]

How does Arrow (c++) decide which function registry to look into? And in which order in case there would be conflicting names?

The created registry on Python is passed into the C++ when we register the function. It is added as an option.

Checking the code, it seems that pc.call_function(..) also only looks in the default, global function registry. So in practice, how useful is it that you can register a UDF in another registry? How does this then actually get used? (how do you call it manually with pc.call_function(..), how does it get resolved if you reference this kernel in a projection or query?)

I agree with this restriction. I kept this in draft as this part is not yet clear or the choice we have to make when using this.
Also @jorisvandenbossche this was implemented as part of a discussion went on the JIRA itself.

cc @westonpace to gather more thoughts about the usage in Acero, etc.

[amol-]

Closing because it has been untouched for a while, in case it's still relevant feel free to reopen and move it forward 👍