Register get_field by default

[leoyvens]

This makes sense in principle, as all other core udfs are register by default in the context.

It also has a practical use, which is executing logical plans that have field access already de-sugared into get_field invocations.

Are these changes tested?

Yes

Are there any user-facing changes?

This is technically user-facing, though there aren't a lot of use cases for actually calling get_field.

[jayzhan211]

I think the reason that get_field is not enabled by default is that is it indirectly called by syntax. For example, struct()['a'] call get_field

datafusion/datafusion/functions-nested/src/planner.rs

Lines 145 to 147 in 7a1a23d

	GetFieldAccess::NamedStructField { name } => {
	Ok(PlannerResult::Planned(get_field(expr, name)))
	}

[leoyvens]

That's right, this is not for end-users writing SQL queries. My use case is deserializing and executing logical plans that have s['a'] syntax desugared into get_field(s, 'a'), for that it is useful to have get_field registered in the context.

[alamb]

This came up in the ASF slack channel too: https://the-asf.slack.com/archives/C04RJ0C85UZ/p1723558402071579

Thus that is two users so I think this is a good idea to add

[alamb]

Thank you @leoyvens and @jayzhan211

This change makes sense to me given the usecase.

[alamb]

It might help to add the context of why this function is exported. Perhaps something like

Suggested change

	get_field(),
	// Note most users invoke `get_field` indirectly via field access syntax
	// like `my_struct_col['field_name']` results in a call to `get_field(my_struct_col, "field_name")`
	// It is also exposed directly for use cases such as serializing / deserializing plans with
	// the field access desugared to calls to `get_field`
	get_field(),

[alamb]

Added in follow on PR: #11996