Arrays cannot hold concretely typed children

[gatesn]

What happened?

At least FSSTArray, possibly others, hold concretely typed child arrays. In this case, FSST holds a VarBinArray for its codes.

This makes the with_children function error when swapping the child out for something that isn't a VarBinArray.

Unfortunately, we're a little stuck. We could split this up into offsets / data children, but that's a breaking change to the serde format. For now we may have to just convert the new child array back to VarBinArray, at whatever cost.

A fix would be to deprecate FSST, and implement a reduce function to replace itself with FSST2. But we should really work out how we to support GPU decoding before doing this.

Steps to reproduce

FSSTArray::with_children

Environment

Develop

Additional context

No response

[robert3005]

As long as we can read old values this is fine to change. I guess we just require binary dtype array. And change the thing to hold arrayref. Old versions wouldn’t be able to read new ones but we don’t guarantee it yet