GH-22342: [C++] [Documentation] add discussion of Union.typeIds to Layout.rst#50097
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GH-22342: [C++] [Documentation] add discussion of Union.typeIds to Layout.rst#50097thisisnic wants to merge 1 commit into
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Pull request overview
This PR updates the Arrow format documentation to clarify how union type ids relate to union children and the on-wire “types buffer”, improving reader understanding of cases where type ids don’t match child array indices.
Changes:
- Adds an explicit explanation that union child types have assigned type ids which may differ from child array indices.
- Updates the dense union “Types buffer” description to reference the above mapping and avoid implying ids must be contiguous/zero-based.
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| Each child type in a union has a type id (an 8-bit signed integer) | ||
| that identifies it. These type ids are not necessarily the same as the | ||
| index of the corresponding child array. For example, a union of two types | ||
| might assign type ids 5 and 7 rather than 0 and 1. The mapping from type | ||
| ids to child arrays is part of the union type definition. |
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| * Types buffer: A buffer of 8-bit signed integers, indicating the type | ||
| id of each slot. Note that these type ids are not necessarily the | ||
| same as the child array index (see above). A union with more than 128 | ||
| possible types can be modeled as a union of unions. |
| union can have a value chosen from these types. The types are named | ||
| like a struct's fields, and the names are part of the type metadata. | ||
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| Each child type in a union has a type id (an 8-bit signed integer) |
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