wgtmac commented on code in PR #43995:
URL: https://github.com/apache/arrow/pull/43995#discussion_r1820982299
##########
cpp/src/parquet/arrow/schema.cc:
##########
@@ -681,6 +681,10 @@ Status ListToSchemaField(const GroupNode& group, LevelInfo
current_levels,
// List of primitive type
RETURN_NOT_OK(
NodeToSchemaField(*list_group.field(0), current_levels, ctx, out,
child_field));
+ } else if (list_group.field_count() == 1 &&
list_group.field(0)->is_repeated()) {
Review Comment:
The rule (3) of backward-compatibility rules is that `If the repeated field
is a group with one field and is named either array or uses the LIST-annotated
group's name with _tuple appended then the repeated type is the element type
and elements are required.`. It says that **the repeated type is the element
type**.
```
optional group my_list (LIST) {
repeated group array {
required binary str (STRING);
};
}
```
So for the schema you've just mentioned above, its element type is `group
array { required binary str (STRING); }` which perfectly resolves to
`OneTuple<String>`.
```
optional group a (LIST) {
repeated group array (LIST) {
repeated int32 array;
}
}
```
However, for the schema I've mentioned in this issue, its element type is
`group array (LIST) { repeated int32 array; }` and it perfectly resolves to
`List<int32>` according to rule (1) which is `If the repeated field is not a
group, then its type is the element type and elements are required.`.
The parquet-java implementation has interpreted this case in the same way:
https://github.com/apache/parquet-java/blob/42cf31c0fbe4f000d4ddb1e1092c6634989ea3ca/parquet-avro/src/main/java/org/apache/parquet/avro/AvroSchemaConverter.java#L588
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