replace HashTableLookupExpr with lit(true) in proto serialization (apache#19300)

apache#18393 introduced a new expression that can't be serialized. This causes
*errors* when serializing now, and would break any users using joins +
protobuf.

Author: adriangb

datafusion/physical-plan/src/joins/hash_join/mod.rs

@@ -18,6 +18,7 @@
 //! [`HashJoinExec`] Partitioned Hash Join Operator
 
 pub use exec::HashJoinExec;
+pub use partitioned_hash_eval::HashTableLookupExpr;
 
 mod exec;
 mod inlist_builder;

datafusion/physical-plan/src/joins/hash_join/partitioned_hash_eval.rs

@@ -42,7 +42,7 @@ use crate::{hash_utils::create_hashes, joins::utils::JoinHashMapType};
 /// This is used for:
 /// - Computing routing hashes (with RepartitionExec's 0,0,0,0 seeds)
 /// - Computing lookup hashes (with HashJoin's 'J','O','I','N' seeds)
-pub(super) struct HashExpr {
+pub struct HashExpr {
     /// Columns to hash
     on_columns: Vec<PhysicalExprRef>,
     /// Random state for hashing
@@ -179,7 +179,11 @@ impl HashTableLookupExpr {
     /// * `hash_expr` - Expression that computes hash values
     /// * `hash_map` - Hash table to check membership
     /// * `description` - Description for debugging
-    pub(super) fn new(
+    ///
+    /// # Note
+    /// This is public for internal testing purposes only and is not
+    /// guaranteed to be stable across versions.
+    pub fn new(
         hash_expr: PhysicalExprRef,
         hash_map: Arc<dyn JoinHashMapType>,
         description: String,

datafusion/physical-plan/src/joins/mod.rs

@@ -20,7 +20,7 @@
 use arrow::array::BooleanBufferBuilder;
 pub use cross_join::CrossJoinExec;
 use datafusion_physical_expr::PhysicalExprRef;
-pub use hash_join::HashJoinExec;
+pub use hash_join::{HashJoinExec, HashTableLookupExpr};
 pub use nested_loop_join::NestedLoopJoinExec;
 use parking_lot::Mutex;
 // Note: SortMergeJoin is not used in plans yet
@@ -37,7 +37,11 @@ mod symmetric_hash_join;
 pub mod utils;
 
 mod join_filter;
-mod join_hash_map;
+/// Hash map implementations for join operations.
+///
+/// Note: This module is public for internal testing purposes only
+/// and is not guaranteed to be stable across versions.
+pub mod join_hash_map;
 
 #[cfg(test)]
 pub mod test_utils;

datafusion/proto/src/physical_plan/to_proto.rs

@@ -41,6 +41,7 @@ use datafusion_physical_plan::expressions::{
     BinaryExpr, CaseExpr, CastExpr, Column, InListExpr, IsNotNullExpr, IsNullExpr,
     Literal, NegativeExpr, NotExpr, TryCastExpr, UnKnownColumn,
 };
+use datafusion_physical_plan::joins::HashTableLookupExpr;
 use datafusion_physical_plan::udaf::AggregateFunctionExpr;
 use datafusion_physical_plan::windows::{PlainAggregateWindowExpr, WindowUDFExpr};
 use datafusion_physical_plan::{Partitioning, PhysicalExpr, WindowExpr};
@@ -227,6 +228,30 @@ pub fn serialize_physical_expr(
     let value = snapshot_physical_expr(Arc::clone(value))?;
     let expr = value.as_any();
 
+    // HashTableLookupExpr is used for dynamic filter pushdown in hash joins.
+    // It contains an Arc<dyn JoinHashMapType> (the build-side hash table) which
+    // cannot be serialized - the hash table is a runtime structure built during
+    // execution on the build side.
+    //
+    // We replace it with lit(true) which is safe because:
+    // 1. The filter is a performance optimization, not a correctness requirement
+    // 2. lit(true) passes all rows, so no valid rows are incorrectly filtered out
+    // 3. The join itself will still produce correct results, just without the
+    //    benefit of early filtering on the probe side
+    //
+    // In distributed execution, the remote worker won't have access to the hash
+    // table anyway, so the best we can do is skip this optimization.
+    if expr.downcast_ref::<HashTableLookupExpr>().is_some() {
+        let value = datafusion_proto_common::ScalarValue {
+            value: Some(datafusion_proto_common::scalar_value::Value::BoolValue(
+                true,
+            )),
+        };
+        return Ok(protobuf::PhysicalExprNode {
+            expr_type: Some(protobuf::physical_expr_node::ExprType::Literal(value)),
+        });
+    }
+
     if let Some(expr) = expr.downcast_ref::<Column>() {
         Ok(protobuf::PhysicalExprNode {
             expr_type: Some(protobuf::physical_expr_node::ExprType::Column(

datafusion/proto/tests/cases/roundtrip_physical_plan.rs

@@ -79,8 +79,8 @@ use datafusion::physical_plan::expressions::{
 };
 use datafusion::physical_plan::filter::FilterExec;
 use datafusion::physical_plan::joins::{
-    HashJoinExec, NestedLoopJoinExec, PartitionMode, SortMergeJoinExec,
-    StreamJoinPartitionMode, SymmetricHashJoinExec,
+    HashJoinExec, HashTableLookupExpr, NestedLoopJoinExec, PartitionMode,
+    SortMergeJoinExec, StreamJoinPartitionMode, SymmetricHashJoinExec,
 };
 use datafusion::physical_plan::limit::{GlobalLimitExec, LocalLimitExec};
 use datafusion::physical_plan::placeholder_row::PlaceholderRowExec;
@@ -116,6 +116,7 @@ use datafusion_expr::{
 use datafusion_functions_aggregate::average::avg_udaf;
 use datafusion_functions_aggregate::nth_value::nth_value_udaf;
 use datafusion_functions_aggregate::string_agg::string_agg_udaf;
+use datafusion_physical_plan::joins::join_hash_map::JoinHashMapU32;
 use datafusion_proto::physical_plan::{
     AsExecutionPlan, DefaultPhysicalExtensionCodec, PhysicalExtensionCodec,
 };
@@ -2327,3 +2328,48 @@ async fn roundtrip_async_func_exec() -> Result<()> {
 
     Ok(())
 }
+
+/// Test that HashTableLookupExpr serializes to lit(true)
+///
+/// HashTableLookupExpr contains a runtime hash table that cannot be serialized.
+/// The serialization code replaces it with lit(true) which is safe because
+/// it's a performance optimization filter, not a correctness requirement.
+#[test]
+fn roundtrip_hash_table_lookup_expr_to_lit() -> Result<()> {
+    // Create a simple schema and input plan
+    let schema = Arc::new(Schema::new(vec![Field::new("col", DataType::Int64, false)]));
+    let input = Arc::new(EmptyExec::new(schema.clone()));
+
+    // Create a HashTableLookupExpr - it will be replaced with lit(true) during serialization
+    let hash_map = Arc::new(JoinHashMapU32::with_capacity(0));
+    let hash_expr: Arc<dyn PhysicalExpr> = Arc::new(Column::new("col", 0));
+    let lookup_expr: Arc<dyn PhysicalExpr> = Arc::new(HashTableLookupExpr::new(
+        hash_expr,
+        hash_map,
+        "test_lookup".to_string(),
+    ));
+
+    // Create a filter with the lookup expression
+    let filter = Arc::new(FilterExec::try_new(lookup_expr, input)?);
+
+    // Serialize
+    let ctx = SessionContext::new();
+    let codec = DefaultPhysicalExtensionCodec {};
+    let proto: protobuf::PhysicalPlanNode =
+        protobuf::PhysicalPlanNode::try_from_physical_plan(filter.clone(), &codec)
+            .expect("serialization should succeed");
+
+    // Deserialize
+    let result: Arc<dyn ExecutionPlan> = proto
+        .try_into_physical_plan(&ctx.task_ctx(), &codec)
+        .expect("deserialization should succeed");
+
+    // The deserialized plan should have lit(true) instead of HashTableLookupExpr
+    // Verify the filter predicate is a Literal(true)
+    let result_filter = result.as_any().downcast_ref::<FilterExec>().unwrap();
+    let predicate = result_filter.predicate();
+    let literal = predicate.as_any().downcast_ref::<Literal>().unwrap();
+    assert_eq!(*literal.value(), ScalarValue::Boolean(Some(true)));
+
+    Ok(())
+}