pitrou commented on code in PR #50839:
URL: https://github.com/apache/arrow/pull/50839#discussion_r4193410130
##########
docs/source/format/Columnar.rst:
##########
@@ -864,6 +864,12 @@ A union is defined by an ordered sequence of types; each
slot in the
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.
+Each child type in a union has a type id (an 8-bit signed integer)
+that identifies it in the types buffer. By default, these type ids
+correspond directly to the child array indices (0, 1, ...), but an optional
+``typeIds`` metadata array can provide an explicit mapping from child array
+offset to type id.
Review Comment:
```suggestion
Each child type in a union has a logical type id (an 8-bit signed integer)
that identifies it in the types buffer. By default, these type ids
correspond directly to the child array indices (0, 1, ...), but an optional
``typeIds`` metadata array can provide an explicit mapping from child array
index to type id.
```
##########
docs/source/format/Columnar.rst:
##########
@@ -878,10 +884,10 @@ Dense union represents a mixed-type array with 5 bytes of
overhead for
each value. Its physical layout is as follows:
* One child array for each type
-* Types buffer: A buffer of 8-bit signed integers. Each type in the
- union has a corresponding type id whose values are found in this
- buffer. A union with more than 128 possible types can be modeled as
- a union of unions.
+* 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.
Review Comment:
```suggestion
* Types buffer: A buffer of 8-bit signed integers, indicating the logical
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.
```
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