add option in Unnest

[wizardxz]

No description provided.

[findepi]

Is this a cherry pick from upstream?

[wizardxz]

Is this a cherry pick from upstream?

I wrote this, I need this diff to differentiate databricks explode(not expanding struct) and inline(expanding struct)

[findepi]

Is this upstreamable?

We should avoid any permanent changes on the fork. If this is not upstreamable, let's find an alternative approach. For example, can this be modeled as a UDF call or UDF + unnest?

For example, of unnest always expands a struct, modeling non-expanding struct could be as easy as wrapping the data in another layer of struct before calling unnest.

[wizardxz]

Is this upstreamable?
yes

We should avoid any permanent changes on the fork.
Totally agree, this PR is just a demo for @vgapeyev

can this be modeled as a UDF call or UDF + unnest?
Yes
I cloned a lot of code from datafusion for post-processing Expr::Unnest
https://github.com/dbt-labs/fs/blob/zhong/databricks_table_functions/crates/sdf-frontend/src/sql/common.rs#L3167
If we want to model unnest as UDF, then we need to post-processing UDF
The good thing is if we model as UDF, we can add as many metadata(e.g. inline_struct_fields) as we want
But I still prefer upstreaming this PR, I think datafusion eventually need this.

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