fix: coalesce schema issues

[mesejo]

Which issue does this PR close?

Closes #12307.

Are these changes tested?

Yes

Are there any user-facing changes?

No

[jayzhan211]

Thanks @mesejo, I think overall looks good to me

[alamb]

Thanks @mesejo and @jayzhan211

cc @findepi who I believe is also working in this area / thinking about functions

[alamb]

It seems to me that moving the signature from a data driven description (aka describe "what" is needed and letting some other code compute if the given arguments match that signature), this PR is moving many of the functions towards more functional (each function has to implement its own custom coercion, likely resulting in significant duplication).

What do you think (perhaps as a follow on PR) of adding DataType::Null support to the Signature calculations somehow rather than inlining / duplicating the coercion logic?

Maybe something like

Signature::allow_null(..)

that would support automatically coercing arguments from null?

Or maybe we should always support coercing Null to any type

[jayzhan211]

Alternative signature like Signature::String, similar to Signature::numeric that includes converting null to string too?

[alamb]

I am not sure -- I was just reacting that this "handle null" pattern seems common and it seems like this approach will require custom coerce logic for all functions 🤔

[findepi]

Null to T coercion needs to be handled elsewhere anyway (eg when computing type of a UNION, etc.).
We can free functions from having to bother about coercions at all and let the engine calculate coercions when building the logical plan.

This is actually super fundamental for DataFusion vision as a composable query engine. Coercion rules are very implementation-specific. If we had functions spiced up with coercions inside them, that would make those functions non-reusable.

cc @wizardxz @sadboy

[alamb]

100%

It seems to me like Signature is supposed to communicate what types the function implementation has a native implementation for and the coercion of whatever the user provided doesn't match one of the supported types