[HSTACK] [DF] - prune files scanned based on pushed down limit

Author: aditanase

crates/core/src/delta_datafusion/mod.rs

@@ -594,31 +594,64 @@ impl<'a> DeltaScanBuilder<'a> {
                 (files, files_scanned, 0)
             }
             None => {
-                if let Some(predicate) = &logical_filter {
-                    let pruning_predicate =
-                        PruningPredicate::try_new(predicate.clone(), logical_schema.clone())?;
-                    let files_to_prune = pruning_predicate.prune(self.snapshot)?;
-                    let mut files_pruned = 0usize;
-                    let files = self
+                // early return in case we have no push down filters or limit
+                if logical_filter.is_none() && self.limit.is_none() {
+                    let files = self.snapshot.file_actions()?;
+                    let files_scanned = files.len();
+                    (files, files_scanned, 0)
+                } else {
+                    let num_containers = self.snapshot.num_containers();
+
+                    let files_to_prune = if let Some(predicate) = &logical_filter {
+                        let pruning_predicate =
+                            PruningPredicate::try_new(predicate.clone(), logical_schema.clone())?;
+                        pruning_predicate.prune(self.snapshot)?
+                    } else {
+                        vec![true; num_containers]
+                    };
+
+                    // needed to enforce limit and deal with missing statistics
+                    // rust port of https://github.com/delta-io/delta/pull/1495
+                    let mut pruned_without_stats = vec![];
+                    let mut rows_collected = 0;
+                    let mut files = vec![];
+
+                    for (action, keep) in self
                         .snapshot
                         .file_actions_iter()?
                         .zip(files_to_prune.into_iter())
-                        .filter_map(|(action, keep)| {
-                            if keep {
-                                Some(action.to_owned())
+                    {
+                        // prune file based on predicate pushdown
+                        if keep {
+                            // prune file based on limit pushdown
+                            if let Some(limit) = self.limit {
+                                if let Some(stats) = action.get_stats()? {
+                                    if rows_collected <= limit as i64 {
+                                        rows_collected += stats.num_records;
+                                        files.push(action.to_owned());
+                                    } else {
+                                        break;
+                                    }
+                                } else {
+                                    // some files are missing stats; skipping but storing them
+                                    // in a list in case we can't reach the target limit
+                                    pruned_without_stats.push(action.to_owned());
+                                }
                             } else {
-                                files_pruned += 1;
-                                None
+                                files.push(action.to_owned());
                             }
-                        })
-                        .collect::<Vec<_>>();
+                        }
+                    }
+
+                    if let Some(limit) = self.limit {
+                        if rows_collected < limit as i64 {
+                            files.extend(pruned_without_stats);
+                        }
+                    }
 
                     let files_scanned = files.len();
+                    let files_pruned = num_containers - files_scanned;
                     (files, files_scanned, files_pruned)
-                } else {
-                    let files = self.snapshot.file_actions()?;
-                    let files_scanned = files.len();
-                    (files, files_scanned, 0)
                 }
             }
         };