etseidl commented on code in PR #11157:
URL: https://github.com/apache/arrow-rs/pull/11157#discussion_r4063372748
##########
parquet/src/file/metadata/reader.rs:
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@@ -104,6 +107,114 @@ impl From<bool> for PageIndexPolicy {
}
}
+/// Struct to specify column chunks for which metadata is required.
+///
+/// Column chunks are identified by row group index and column index. This
struct
+/// allows for specifying vertical slices of column chunk data (via
[`Self::columns`]),
+/// horizontal slices (via [`Self::row_groups`]), or the intersection of the
two
+/// (via [`Self::row_groups_and_columns`]).
+///
+/// At present this is only used to select elements of the [Page Index] for
decoding.
+///
+/// # Examples
+///
+/// To select columns 0 and 1 from all row groups:
+/// ```rust
+/// # use parquet::file::metadata::ColumnChunkMask;
+/// let mask = ColumnChunkMask::columns([0, 1]);
+/// ```
+///
+/// To select all columns from row group 2:
+/// ```rust
+/// # use parquet::file::metadata::ColumnChunkMask;
+/// let mask = ColumnChunkMask::row_groups([2]);
+/// ```
+///
+/// To select columns 1 and 3 from row group 0:
+/// ```rust
+/// # use parquet::file::metadata::ColumnChunkMask;
+/// let mask = ColumnChunkMask::row_groups_and_columns([0], [1, 3]);
+/// ```
+///
+/// [Page Index]: https://parquet.apache.org/docs/file-format/pageindex/
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct ColumnChunkMask {
Review Comment:
A note on naming: this was originally `PageIndexSelection`, but it occurred
to me that we might want to use this for the file metadata as well. I changed
it to `RowGroupColumnSelection`, but that was a bit unwieldy. I asked Google
for some naming suggestions, and this name made the most sense to me since the
intersection of row group and column is a column chunk.
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