Unnesting
Version dev

## Examples

``````-- unnest a list, generating 3 rows (1, 2, 3)
SELECT unnest([1, 2, 3]);
-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': 42, 'b': 84});
-- recursive unnest of a list of structs
SELECT unnest([{'a': 42, 'b': 84}, {'a': 100, 'b': NULL}], recursive := true);
``````

The `unnest` function is used to unnest lists or structs by one level. The function can be used as a regular scalar function, but only in the `SELECT` clause. Invoking `unnest` with the `recursive` parameter will unnest lists and structs of multiple levels.

### Unnesting Lists

``````-- unnest a list, generating 3 rows (1, 2, 3)
SELECT unnest([1, 2, 3]);
-- unnest a scalar list, generating 3 rows ((1, 10), (2, 11), (3, NULL))
SELECT unnest([1, 2, 3]), unnest([10, 11]);
-- unnest a scalar list, generating 3 rows ((1, 10), (2, 10), (3, 10))
SELECT unnest([1, 2, 3]), 10;
-- unnest a list column generated from a subquery
SELECT unnest(l) + 10 FROM (VALUES ([1, 2, 3]), ([4, 5])) tbl(l);
-- empty result
SELECT unnest([]);
-- empty result
SELECT unnest(NULL);
``````

Using `unnest` on a list will emit one tuple per entry in the list. When `unnest` is combined with regular scalar expressions, those expressions are repeated for every entry in the list. When multiple lists are unnested in the same `SELECT` clause, the lists are unnested side-by-side. If one list is longer than the other, the shorter list will be padded with `NULL` values.

An empty list and a `NULL` list will both unnest to zero elements.

### Unnesting Structs

``````-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': 42, 'b': 84});
-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': 42, 'b': {'x': 84}});
``````

`unnest` on a struct will emit one column per entry in the struct.

### Recursive Unnest

``````-- unnesting a list of lists recursively, generating 5 rows (1, 2, 3, 4, 5)
SELECT unnest([[1, 2, 3], [4, 5]], recursive := true);
-- unnesting a list of structs recursively, generating two rows of two columns (a, b)
SELECT unnest([{'a': 42, 'b': 84}, {'a': 100, 'b': NULL}], recursive := true);
-- unnesting a struct, generating two columns (a, b)
SELECT unnest({'a': [1, 2, 3], 'b': 88}, recursive := true);
``````

Calling `unnest` with the `recursive` setting will fully unnest lists, followed by fully unnesting structs. This can be useful to fully flatten columns that contain lists within lists, or lists of structs. Note that lists within structs are not unnested.

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