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Unnesting

## 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);

Limit depth of recursive unnest using `max_depth`:

    SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 2);

The `unnest` special 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. The depth of unnesting can be limited using the `max_depth` parameter (which assumes `recursive` unnesting by default).

### 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.

### Setting the Maximum Depth of Unnesting

The `max_depth` parameter allows limiting the maximum depth of recursive unnesting (which is assumed by default and does not have to be specified separately). For example, unnestig to `max_depth` of 2 yields the following:

    SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 2) AS x;

x
[1, 2]
[3, 4]
[5, 6]
[7, 8, 9]
[]
[10, 11]

Meanwhile, unnesting to `max_depth` of 3 results in:

    SELECT unnest([[[1, 2], [3, 4]], [[5, 6], [7, 8, 9], []], [[10, 11]]], max_depth := 3) AS x;

x
1
2
3
4
5
6
7
8
9
10
11

### Keeping Track of List Entry Positions

To keep track of each entry's position within the original list, `unnest` may be combined with `generate_subscripts`:

    SELECT unnest(l) AS x, generate_subscripts(l, 1) AS index
    FROM (VALUES ([1, 2, 3]), ([4, 5])) tbl(l);

x | index
--- | ---
1 | 1
2 | 2
3 | 3
4 | 1
5 | 2

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