Lambda the Ultimate

I'm building a relational language where I wish to treat everything as a relation. The main point is to provide a common core of relational operators (aka: Queries) that work across all. A "relation" here is a table with a header of fields, columns and rows.

I have a container that match directly this, that internally is an array:

Now, I have other containers that are Key/Value like a hashmap and BTreeMap, and single value like Generators, ranges and vectors. Only generators, as being read-only forward-only can be say to output a fixed schema.

You can say a hashmap, for example, is a relation that look like:

And a vector one like

With all of them, I wanna provide the same operations like:

table ?where #age = 18
kv    ?where #key =1 
array ?where #it = 2

Now my dilema is what to do for some operations that add or remove columns, like joins and projection:

table ?select *, #age + 10 as older
kv    ?select *, #value |> String.Upper as name
array ?select *, it * 10 as calc

What could be the best option here?

1- Say that kv/array have fixed schemas and can't grow or shrink their headers

2- Say a KV only have fixed the KEY and the value header is flattened, so it become:

[key, value, name]

The problem is not what OUTPUT. I could just output tuples. Is how REPRESENT the relation if it be more than the KV.

So, If I have 10 columns, what is the best option to put inside a KV? Consider that the relational model allow to rename columns, and add and remove them.

Another problem is that KV like Btree allow to model... trees. If I think instead of Btree as a index behind a table, the language lost the ability of "naturally" model trees. BTW, I wanna to have AGDTs.

P.D: Other potential question is: What kind of container could work alike "table" but also alike hashmaps/trees? How model a type that allow several "backends"?

P.D: Originally here: https://www.reddit.com/r/ProgrammingLanguages/comments/awkdrb/for_a_relational_language_how_treat_kv_and/ but not good answers... so I ask here with a bit of more details.