tobixdev commented on code in PR #20312:
URL: https://github.com/apache/datafusion/pull/20312#discussion_r2987075066
##########
datafusion/expr/src/registry.rs:
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@@ -215,3 +218,281 @@ impl FunctionRegistry for MemoryFunctionRegistry {
self.udwfs.keys().cloned().collect()
}
}
+
+/// A cheaply cloneable pointer to an [ExtensionTypeRegistration].
+pub type ExtensionTypeRegistrationRef = Arc<dyn ExtensionTypeRegistration>;
+
+/// The registration of an extension type. Implementations of this trait are
responsible for
+/// *creating* instances of [`DFExtensionType`] that represent the entire
semantics of an extension
+/// type.
+///
+/// # Why do we need a Registration?
+///
+/// A good question is why this trait is even necessary. Why not directly
register the
+/// [`DFExtensionType`] in a registration?
+///
+/// While this works for extension types without parameters (e.g.,
`arrow.uuid`), it does not work
+/// for more complex extension types that may have another extension type as a
parameter. For
+/// example, consider an extension type `custom.shortened(n)` that aims to
short the pretty-printing
+/// string to `n` characters. Here, `n` is a parameter of the extension type
and should be a field
+/// in the concrete struct that implements the [`DFExtensionType`]. The job of
the registration is
+/// to read the metadata from the field and create the corresponding
[`DFExtensionType`] instance
+/// with the correct `n` set.
Review Comment:
I've not implemented this for now as I couln't find a good formulation that
does not introduce additional potential for confusion. Happy to add it if
someone is good with words.
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