wjones127 commented on code in PR #15184:
URL: https://github.com/apache/arrow/pull/15184#discussion_r1061870977
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cpp/src/parquet/arrow/reader.h:
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@@ -225,7 +226,19 @@ class PARQUET_EXPORT FileReader {
/// \brief Read the given columns into a Table
///
- /// The indicated column indices are relative to the schema
+ /// The indicated column indices are relative to the internal representation
+ /// of the parquet table. For instance :
+ /// 0 foo.bar
+ /// foo.bar.baz 0
+ /// foo.bar.baz2 1
+ /// foo.qux 2
+ /// 1 foo2 3
+ /// 2 foo3 4
+ ///
+ /// i=0 will read foo.bar.baz, i=1 will read only foo.bar.baz2 and so on.
+ /// To retrive all the indices corresponding to the upper level arrow schema,
+ /// one can use manifest().schema_fields and get all the indices of the
leaves
+ /// for each particular upper level field index.
Review Comment:
What do you think of this?
```suggestion
/// Only leaf fields have indices; foo itself doesn't have an index.
/// To get the index for a particular leaf field, one can use
/// manifest().schema_fields to get the top level fields, and then walk the
/// tree to identify the relevant leaf fields and access its column_index.
/// To get the total number of leaf fields, use FileMetadata.num_columns().
```
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