Sketch out a Morselize API

datafusion/datasource-parquet/src/access_plan.rs

@@ -16,9 +16,10 @@
 // under the License.
 
 use crate::sort::reverse_row_selection;
-use datafusion_common::{Result, assert_eq_or_internal_err};
+use datafusion_common::{Result, assert_eq_or_internal_err, internal_datafusion_err};
 use parquet::arrow::arrow_reader::{RowSelection, RowSelector};
 use parquet::file::metadata::{ParquetMetaData, RowGroupMetaData};
+use std::collections::VecDeque;
 
 /// A selection of rows and row groups within a ParquetFile to decode.
 ///
@@ -396,6 +397,80 @@ impl PreparedAccessPlan {
 
         Ok(self)
     }
+
+    /// Split this access plan into one plan per selected row group.
+    ///
+    /// The returned plans preserve the current row-group ordering. If
+    /// `row_selection` is present, it is partitioned so each returned plan
+    /// contains only the selection entries for its single row group.
+    pub(crate) fn into_single_row_group_plans(
+        self,
+        file_metadata: &ParquetMetaData,
+    ) -> Result<Vec<Self>> {
+        let Self {
+            row_group_indexes,
+            row_selection,
+        } = self;
+
+        let Some(row_selection) = row_selection else {
+            return Ok(row_group_indexes
+                .into_iter()
+                .map(|row_group_index| Self {
+                    row_group_indexes: vec![row_group_index],
+                    row_selection: None,
+                })
+                .collect());
+        };
+
+        let mut selectors: VecDeque<RowSelector> =
+            Vec::<RowSelector>::from(row_selection).into();
+        let mut plans = Vec::with_capacity(row_group_indexes.len());
+
+        for row_group_index in row_group_indexes {
+            let mut remaining_rows =
+                file_metadata.row_groups()[row_group_index].num_rows() as usize;
+            let mut row_group_selectors = Vec::new();
+
+            while remaining_rows > 0 {
+                let selector = selectors.pop_front().ok_or_else(|| {
+                    internal_datafusion_err!(
+                        "PreparedAccessPlan row selection ended before row group {row_group_index} was fully described"
+                    )
+                })?;
+
+                let rows_for_group = selector.row_count.min(remaining_rows);
+                row_group_selectors.push(if selector.skip {
+                    RowSelector::skip(rows_for_group)
+                } else {
+                    RowSelector::select(rows_for_group)
+                });
+
+                if selector.row_count > rows_for_group {
+                    let remaining_selector_rows = selector.row_count - rows_for_group;
+                    selectors.push_front(if selector.skip {
+                        RowSelector::skip(remaining_selector_rows)
+                    } else {
+                        RowSelector::select(remaining_selector_rows)
+                    });
+                }
+
+                remaining_rows -= rows_for_group;
+            }
+
+            plans.push(Self {
+                row_group_indexes: vec![row_group_index],
+                row_selection: Some(row_group_selectors.into()),
+            });
+        }
+
+        if !selectors.is_empty() {
+            return Err(internal_datafusion_err!(
+                "PreparedAccessPlan row selection had leftover selectors after splitting by row group"
+            ));
+        }
+
+        Ok(plans)
+    }
 }
 
 #[cfg(test)]

datafusion/datasource-parquet/src/mod.rs

@@ -37,6 +37,7 @@ mod writer;
 pub use access_plan::{ParquetAccessPlan, RowGroupAccess};
 pub use file_format::*;
 pub use metrics::ParquetFileMetrics;
+pub use opener::ParquetMorselizer;
 pub use page_filter::PagePruningAccessPlanFilter;
 pub use reader::*; // Expose so downstream crates can use it
 pub use row_filter::build_row_filter;