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refactor: Simplify and optimize Parquet projection planning for nested casts #24522

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refactor: Simplify and optimize Parquet projection planning for nested casts #24522

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160 changes: 64 additions & 96 deletions datafusion/datasource-parquet/src/projection_read_plan.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -571,6 +571,14 @@ pub(crate) fn build_projection_read_plan(
read_plan
}

/// Leaf selection accumulated for one projected root column.
enum RootRead {
/// Decode every leaf and preserve the physical Arrow field.
Full,
/// Decode these leaf offsets relative to the start of this root.
Partial(BTreeSet<usize>),
}

/// Builds a [`ParquetReadPlan`] when at least one projected root column is
/// consumed through a cast to a narrower nested type.
///
Expand All @@ -595,21 +603,20 @@ fn build_read_plan_with_cast_clipping(
struct_accesses: &[StructFieldAccess],
cast_accesses: &[CastColumnAccess],
) -> ParquetReadPlan {
let whole_roots: BTreeSet<usize> = whole_root_indices.iter().copied().collect();
// Every referenced root's Parquet leaves, grouped in one pass over the
// schema descriptor rather than one `leaf_indices_for_roots` scan per
// root (this function may look up several roots).
let leaves_by_root = leaves_grouped_by_root(schema_descr);

// Root -> relative leaf offsets required by every narrowing cast on that
// root. A set makes repeated and overlapping targets a natural union.
let mut kept_offsets_by_root: BTreeMap<usize, BTreeSet<usize>> = BTreeMap::new();
// Roots with a cast access that must fall back to a full read.
let mut fallback_roots: BTreeSet<usize> = BTreeSet::new();
// Keep one decision per root. The ordered map also determines the output
// schema order, which must match the Parquet reader's root order.
let mut root_reads: BTreeMap<usize, RootRead> = whole_root_indices
.iter()
.map(|root| (*root, RootRead::Full))
.collect();

for access in cast_accesses {
let root = access.root_index;
if whole_roots.contains(&root) || fallback_roots.contains(&root) {
if matches!(root_reads.get(&root), Some(RootRead::Full)) {
continue;
}

Expand All @@ -621,130 +628,91 @@ fn build_read_plan_with_cast_clipping(
// arrow schema). If not, never risk a wrong mask: read the whole
// root.
if root_leaves.len() != count_leaves(physical_type) {
fallback_roots.insert(root);
root_reads.insert(root, RootRead::Full);
continue;
}

match clip_for_cast(physical_type, &access.target_type) {
Some((kept_offsets, _pruned_type)) => {
kept_offsets_by_root
if let RootRead::Partial(offsets) = root_reads
.entry(root)
.or_default()
.extend(kept_offsets);
.or_insert_with(|| RootRead::Partial(BTreeSet::new()))
{
offsets.extend(kept_offsets);
}
}
// Nothing prunable for this cast: every leaf is consumed.
None => {
kept_offsets_by_root.remove(&root);
fallback_roots.insert(root);
root_reads.insert(root, RootRead::Full);
}
}
}

// Add leaves reached through `get_field` to cast roots. The resolver
// returns absolute Parquet leaf indices; convert them back to offsets in
// their root so they can share the same union as cast clipping.
// Add every `get_field` root before resolving leaves. If an access matches
// no leaf, finalization safely falls back to a full read for that root.
Comment on lines +651 to +652

@adriangb adriangb Aug 20, 2026

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This isn't a pure refactor — it fixes a bug. Routing get_field-only roots through type_for_leaf_subset instead of build_filter_schema changes what happens when an access path matches no Parquet leaf. With a cast on root a and a b['nonexistent'] access on root b:

projected schema mask
before a: Struct<p>, b: Struct() [0]
after a: Struct<p>, b: Struct<m,n> [0, 2, 3]

The old path emitted an empty struct with zero leaves selected, a schema the reader can't produce. The fallback here is correct, but nothing tests it; the private access() helper makes a regression test ~20 lines. Worth calling out in the description too, so this isn't reviewed as a no-op.

While here: the doc comment at L594 still says get_field-only roots behave "as before", which is no longer true.

@mbutrovich mbutrovich Aug 20, 2026

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If it fixes a bug can we get a regression test?

for access in struct_accesses {
root_reads
.entry(access.root_index)
.or_insert_with(|| RootRead::Partial(BTreeSet::new()));
}

// The resolver returns absolute Parquet leaf indices. Convert each selected
// leaf to a root-relative offset so casts and field accesses share one union.
let struct_access_tree = StructAccessTree::from_accesses(struct_accesses);
for leaf in resolve_struct_field_leaves(&struct_access_tree, schema_descr) {
let root = schema_descr.get_column_root_idx(leaf);
if !kept_offsets_by_root.contains_key(&root) {
let Some(RootRead::Partial(offsets)) = root_reads.get_mut(&root) else {
continue;
}
};
let Some(offset) = leaves_by_root
.get(&root)
.and_then(|root_leaves| root_leaves.binary_search(&leaf).ok())
else {
kept_offsets_by_root.remove(&root);
fallback_roots.insert(root);
continue;
};
kept_offsets_by_root
.get_mut(&root)
.expect("root presence checked above")
.insert(offset);
}

// Derive the reader's one emitted Arrow type from each merged leaf set.
// Any unsupported partial wrapper retains the total fallback guarantee.
let mut clipped_by_root: BTreeMap<usize, (Vec<usize>, DataType)> = BTreeMap::new();
for (root, kept_offsets) in kept_offsets_by_root {
if fallback_roots.contains(&root) {
continue;
}
let physical_type = file_schema.field(root).data_type();
let root_leaves = leaves_by_root.get(&root).map_or(&[][..], Vec::as_slice);
let kept_offsets = kept_offsets.into_iter().collect::<Vec<_>>();
let Some(pruned_type) = type_for_leaf_subset(physical_type, &kept_offsets) else {
fallback_roots.insert(root);
root_reads.insert(root, RootRead::Full);
continue;
};
let absolute = kept_offsets
.into_iter()
.map(|offset| root_leaves[offset])
.collect();
clipped_by_root.insert(root, (absolute, pruned_type));
offsets.insert(offset);
}

// `get_field` accesses on roots not already read in full (as a whole
// column, or as a cast that fell back) keep the existing (non-cast) leaf
// resolution.
let get_field_accesses: Vec<StructFieldAccess> = struct_accesses
.iter()
.filter(|a| {
!whole_roots.contains(&a.root_index)
&& !fallback_roots.contains(&a.root_index)
&& !clipped_by_root.contains_key(&a.root_index)
})
.cloned()
.collect();

let mut leaf_indices: Vec<usize> = Vec::new();
let mut fields: BTreeMap<usize, Arc<Field>> = BTreeMap::new();

for root in whole_roots.iter().chain(fallback_roots.iter()) {
// A root with no parquet leaves contributes nothing to the mask;
// `ProjectionMask::roots` handles that case the same way, so match it
// rather than indexing and panicking.
if let Some(leaves) = leaves_by_root.get(root) {
leaf_indices.extend(leaves.iter().copied());
let mut fields = Vec::with_capacity(root_reads.len());
for (root, read) in root_reads {
let field = file_schema.field(root);
let root_leaves = leaves_by_root.get(&root).map_or(&[][..], Vec::as_slice);
match read {
RootRead::Partial(offsets)
if root_leaves.len() == count_leaves(field.data_type()) =>

@adriangb adriangb Aug 20, 2026

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This guard is redundant for cast roots (already checked in the loop above) but is newly applied to get_field-only roots, which never had it. This is a safe direction since it just falls back to a full read. But it's a silent narrowing; worth a comment saying so.

{
let offsets = offsets.into_iter().collect::<Vec<_>>();
if let Some(projected_type) =
type_for_leaf_subset(field.data_type(), &offsets)
{
leaf_indices
.extend(offsets.into_iter().map(|offset| root_leaves[offset]));
fields.push(field_with_type(field, projected_type));

@adriangb adriangb Aug 20, 2026

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field_with_type preserves root field metadata; assemble_read_plan still goes through build_filter_schema, which rebuilds roots with Field::new(...) and drops it. Same access, same leaves, same type:

cast path : b = Struct("m": Int32) meta={"k": "v"}
plain path: b = Struct("m": Int32) meta={}

So a column's projected field now depends on whether an unrelated column carries a narrowing cast. This side is the right behavior: worth switching build_filter_schema to field_with_type too, here or as a follow-up.

continue;
}
}
RootRead::Full | RootRead::Partial(_) => {}
}
fields.insert(*root, Arc::new(file_schema.field(*root).clone()));
}

for (&root, (kept, pruned_type)) in &clipped_by_root {
leaf_indices.extend(kept.iter().copied());
fields.insert(
root,
field_with_type(file_schema.field(root), pruned_type.clone()),
);
// Full reads and unsupported/empty partial reads preserve the physical
// field. A root with no Parquet leaves contributes only its Arrow field.
leaf_indices.extend(root_leaves.iter().copied());
fields.push(Arc::new(field.clone()));
}

if !get_field_accesses.is_empty() {
let get_field_tree = StructAccessTree::from_accesses(&get_field_accesses);
leaf_indices.extend(resolve_struct_field_leaves(&get_field_tree, schema_descr));
let get_field_schema = build_filter_schema(file_schema, &[], &get_field_tree);
let get_field_roots: BTreeSet<usize> =
get_field_accesses.iter().map(|a| a.root_index).collect();
// `build_filter_schema` emits one field per accessed root in
// ascending root order, which is the order `get_field_roots` iterates
// in, so the two line up positionally. Pairing them beats looking each
// one up by name: no repeated linear scans, and no ambiguity if two
// roots happen to share a name.
debug_assert_eq!(get_field_roots.len(), get_field_schema.fields().len());
for (root, field) in get_field_roots.iter().zip(get_field_schema.fields()) {
fields.insert(*root, Arc::clone(field));
}
}

leaf_indices.sort_unstable();
leaf_indices.dedup();
// `root_reads` visits roots in schema order, every root's leaves were
// collected in descriptor order, and partial offsets are a `BTreeSet`.
// Therefore the final mask is already sorted and deduplicated.
debug_assert!(leaf_indices.windows(2).all(|pair| pair[0] < pair[1]));

@adriangb adriangb Aug 20, 2026

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Does this even matter?ProjectionMask::leaves just flips booleans in a vec![false; num_columns], so order and duplicates are irrelevant. Dropping the sort_unstable/dedup is fine regardless. The invariant that does carry weight is that fields is pushed in ascending root order to match the reader's output. Consider asserting/documenting that instead?


ParquetReadPlan {
projection_mask: ProjectionMask::leaves(
schema_descr,
leaf_indices.iter().copied(),
),
projected_schema: Arc::new(Schema::new_with_metadata(
fields.into_values().collect::<Vec<_>>(),
fields,
file_schema.metadata().clone(),
)),
}
Expand Down