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Fix structs column description in dev docs #8318

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May 25, 2021
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Fix structs column description in dev docs #8318

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23 changes: 10 additions & 13 deletions cpp/docs/DEVELOPER_GUIDE.md
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Expand Up @@ -964,21 +964,18 @@ this compound column representation of strings.

## Structs columns

Structs are represented similarly to lists, except that they have multiple child data columns.
The parent column's type is `STRUCT` and contains no data, but its size represents the number of
structs in the column, and its null mask represents the validity of each struct element. The parent
has `N + 1` children, where `N` is the number of fields in the struct.
A struct is a nested data type with a set of child columns each representing an individual field
of a logical struct. Field names are not represented.

1. A non-nullable column of `INT32` elements that indicates the offset to the beginning of each
struct in each dense column of elements.
2. For each field, a column containing the actual field data and optional null mask for all elements
of all the structs packed together.
With this representation, `child[0][offsets[i]]` is the first field of struct `i`,
`child[1][offsets[i]]` is the second field of struct `i`, etc.
A structs column with `N` fields has `N` children. Each child is a column storing all the data
of a single field packed column-wise, with an optional null mask. The parent column's type is
`STRUCT` and contains no data, its size represents the number of struct rows in the column, and its
null mask represents the validity of each struct element.

With this representation, `child[0][10]` is row 10 of the first field of the struct, `child[1][42]`
is row 42 of the second field of the struct.

As defined in the [Apache Arrow specification](https://arrow.apache.org/docs/format/Columnar.html#struct-layout),
in addition to the struct column's null mask, each struct field column has its own optional null
Notice that in addition to the struct column's null mask, each struct field column has its own optional null
mask. A struct field's validity can vary independently from the corresponding struct row. For
instance, a non-null struct row might have a null field. However, the fields of a null struct row
are deemed to be null as well. For example, consider a struct column of type
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