[FriendlySQL] Unpacked COLUMNS() Expression

[Tishj]

This PR adds the *COLUMNS(...) expression that changes the behavior of the existing COLUMNS(...) expression.
Instead of expanding vertically, instead the list expands horizontally.

To explain this in more detail, here's some SQL:

COLUMNS behavior

select COALESCE(COLUMNS(*)) from (select NULL, 2, 3) t(a, b, c)

When the COLUMNS expression is expanded, the resulting query becomes:

select COALESCE(a) a, COALESCE(b) b, COALESCE(c) c from (select NULL, 2, 3) t(a, b, c)

The result of this query is:

┌───────┬───────┬───────┐
│   a   │   b   │   c   │
│ int32 │ int32 │ int32 │
├───────┼───────┼───────┤
│       │     2 │     3 │
└───────┴───────┴───────┘

*COLUMNS behavior

select COALESCE(*COLUMNS(*)) from (select NULL, 2, 3) t(a, b, c)

When the *COLUMNS expression is expanded, the resulting query becomes:

select COALESCE(a, b, c) from (select NULL, 2, 3) t(a, b, c)

The result of this query is:

┌─────────────────────────┐
│ COALESCE(t.a, t.b, t.c) │
│          int32          │
├─────────────────────────┤
│                       2 │
└─────────────────────────┘

[Mytherin]

Thanks for the PR! Looks good - some comments:

[Mytherin]

Could we add some more tests?

• Combining COLUMNS(*) with *COLUMNS(*)
• Multiple *COLUMNS(*) in the same statement?
• *COLUMNS(*) inside the lambda of a *COLUMNS(*) statement?
• Can we test this with a varargs function, e.g. CONCAT?
• Can we try *COLUMNS(*) + 42?

[Tishj]

I noticed that COLUMNS(COLUMNS(COLUMNS(*))) was allowed, because it wasn't checking for "columns" in the transformer, that now properly errors

This was caused by the check that enables COLUMNS(*), COLUMNS is a StarExpression and * is a StarExpression

When a StarExpression (*) appears in COLUMNS we just merge them.
But that also allowed COLUMNS(COLUMNS(COLUMNS(*)))