adriangb commented on code in PR #19404:
URL: https://github.com/apache/datafusion/pull/19404#discussion_r2658026051
##########
datafusion/physical-expr/src/utils/mod.rs:
##########
@@ -238,6 +239,21 @@ pub fn collect_columns(expr: &Arc<dyn PhysicalExpr>) ->
HashSet<Column> {
columns
}
+pub fn have_unknown_columns(expr: &Arc<dyn PhysicalExpr>) -> bool {
Review Comment:
I think the correct grammar would be `has_unknown_columns(expr)`
##########
datafusion/physical-plan/src/util.rs:
##########
@@ -0,0 +1,312 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use std::sync::Arc;
+
+use datafusion_common::{
+ HashMap,
+ tree_node::{Transformed, TreeNodeRecursion, TreeNodeRewriter},
+};
+use datafusion_physical_expr::{
+ PhysicalExpr,
+ expressions::{Column, UnKnownColumn},
+};
+
+/// Rewrite column references in a physical expr according to a mapping.
Review Comment:
A more detailed docstring (maybe with examples?) would be nice.
##########
datafusion/sqllogictest/test_files/topk.slt:
##########
@@ -383,7 +383,7 @@ physical_plan
03)----ProjectionExec: expr=[__common_expr_1@0 as number_plus, number@1 as
number, __common_expr_1@0 as other_number_plus, age@2 as age]
04)------ProjectionExec: expr=[CAST(number@0 AS Int64) + 1 as __common_expr_1,
number@0 as number, age@1 as age]
05)--------RepartitionExec: partitioning=RoundRobinBatch(4),
input_partitions=1, maintains_sort_order=true
-06)----------DataSourceExec: file_groups={1 group:
[[WORKSPACE_ROOT/datafusion/sqllogictest/test_files/scratch/topk/partial_sorted/1.parquet]]},
projection=[number, age], output_ordering=[number@0 DESC], file_type=parquet
+06)----------DataSourceExec: file_groups={1 group:
[[WORKSPACE_ROOT/datafusion/sqllogictest/test_files/scratch/topk/partial_sorted/1.parquet]]},
projection=[number, age], output_ordering=[number@0 DESC], file_type=parquet,
predicate=DynamicFilter [ empty ]
Review Comment:
I think this PR needs more integration tests - both in
`datafusion/core/tests/parquet/filter_pushdown.rs` and as SLT tests
##########
datafusion/physical-expr/src/utils/mod.rs:
##########
@@ -238,6 +239,21 @@ pub fn collect_columns(expr: &Arc<dyn PhysicalExpr>) ->
HashSet<Column> {
columns
}
+pub fn have_unknown_columns(expr: &Arc<dyn PhysicalExpr>) -> bool {
Review Comment:
This also needs a docstring.
##########
datafusion/physical-plan/src/projection.rs:
##########
@@ -1270,4 +1302,357 @@ mod tests {
);
assert!(stats.total_byte_size.is_exact().unwrap_or(false));
}
+
+ #[test]
+ fn test_filter_pushdown_with_alias() -> Result<()> {
+ let input_schema = Schema::new(vec![Field::new("a", DataType::Int32,
false)]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics::new_unknown(&input_schema),
+ input_schema.clone(),
+ ));
+
+ // project "a" as "b"
+ let projection = ProjectionExec::try_new(
+ vec![ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "b".to_string(),
+ }],
+ input,
+ )?;
+
+ // filter "b > 5"
+ let filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter],
+ &ConfigOptions::default(),
+ )?;
+
+ // Should be converted to "a > 5"
+ // "a" is index 0 in input
+ let expected_filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ assert_eq!(description.self_filters(), vec![vec![]]);
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(
+ format!("{}", pushed_filters[0].predicate),
+ format!("{}", expected_filter)
+ );
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_multiple_aliases() -> Result<()> {
+ let input_schema = Schema::new(vec![
+ Field::new("a", DataType::Int32, false),
+ Field::new("b", DataType::Int32, false),
+ ]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a" as "x", "b" as "y"
+ let projection = ProjectionExec::try_new(
+ vec![
+ ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "x".to_string(),
+ },
+ ProjectionExpr {
+ expr: Arc::new(Column::new("b", 1)),
+ alias: "y".to_string(),
+ },
+ ],
+ input,
+ )?;
+
+ // filter "x > 5"
+ let filter1 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("x", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ // filter "y < 10"
+ let filter2 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("y", 1)),
+ Operator::Lt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter1, filter2],
+ &ConfigOptions::default(),
+ )?;
+
+ // Should be converted to "a > 5" and "b < 10"
+ let expected_filter1 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let expected_filter2 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 1)),
+ Operator::Lt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(pushed_filters.len(), 2);
+ // Note: The order of filters is preserved
+ assert_eq!(
+ format!("{}", pushed_filters[0].predicate),
+ format!("{}", expected_filter1)
+ );
+ assert_eq!(
+ format!("{}", pushed_filters[1].predicate),
+ format!("{}", expected_filter2)
+ );
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_mixed_columns() -> Result<()> {
+ let input_schema = Schema::new(vec![
+ Field::new("a", DataType::Int32, false),
+ Field::new("b", DataType::Int32, false),
+ ]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a" as "x", "b" as "b" (pass through)
+ let projection = ProjectionExec::try_new(
+ vec![
+ ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "x".to_string(),
+ },
+ ProjectionExpr {
+ expr: Arc::new(Column::new("b", 1)),
+ alias: "b".to_string(),
+ },
+ ],
+ input,
+ )?;
+
+ // filter "x > 5"
+ let filter1 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("x", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ // filter "b < 10" (using output index 1 which corresponds to 'b')
+ let filter2 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 1)),
+ Operator::Lt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter1, filter2],
+ &ConfigOptions::default(),
+ )?;
+
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(pushed_filters.len(), 2);
+ // "x" -> "a" (index 0)
+ let expected_filter1 = "a@0 > 5";
+ // "b" -> "b" (index 1)
+ let expected_filter2 = "b@1 < 10";
+
+ assert_eq!(format!("{}", pushed_filters[0].predicate),
expected_filter1);
+ assert_eq!(format!("{}", pushed_filters[1].predicate),
expected_filter2);
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_complex_expression() -> Result<()> {
+ let input_schema = Schema::new(vec![Field::new("a", DataType::Int32,
false)]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a + 1" as "z"
+ let projection = ProjectionExec::try_new(
+ vec![ProjectionExpr {
+ expr: Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Plus,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(1)))),
+ )),
+ alias: "z".to_string(),
+ }],
+ input,
+ )?;
+
+ // filter "z > 10"
+ let filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("z", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter],
+ &ConfigOptions::default(),
+ )?;
+
+ // expand to `a + 1 > 10`
+ let pushed_filters = &description.parent_filters()[0];
+ assert!(matches!(pushed_filters[0].discriminant, PushedDown::Yes));
+ assert_eq!(format!("{}", pushed_filters[0].predicate), "a@0 + 1 > 10");
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_unknown_column() -> Result<()> {
Review Comment:
Can you help me understand how an unknown column fits into the picture? How
do they get created? Why do we need special handling here?
##########
datafusion/physical-plan/src/projection.rs:
##########
@@ -1270,4 +1302,357 @@ mod tests {
);
assert!(stats.total_byte_size.is_exact().unwrap_or(false));
}
+
+ #[test]
+ fn test_filter_pushdown_with_alias() -> Result<()> {
+ let input_schema = Schema::new(vec![Field::new("a", DataType::Int32,
false)]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics::new_unknown(&input_schema),
+ input_schema.clone(),
+ ));
+
+ // project "a" as "b"
+ let projection = ProjectionExec::try_new(
+ vec![ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "b".to_string(),
+ }],
+ input,
+ )?;
+
+ // filter "b > 5"
+ let filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter],
+ &ConfigOptions::default(),
+ )?;
+
+ // Should be converted to "a > 5"
+ // "a" is index 0 in input
+ let expected_filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ assert_eq!(description.self_filters(), vec![vec![]]);
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(
+ format!("{}", pushed_filters[0].predicate),
+ format!("{}", expected_filter)
+ );
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_multiple_aliases() -> Result<()> {
+ let input_schema = Schema::new(vec![
+ Field::new("a", DataType::Int32, false),
+ Field::new("b", DataType::Int32, false),
+ ]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a" as "x", "b" as "y"
+ let projection = ProjectionExec::try_new(
+ vec![
+ ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "x".to_string(),
+ },
+ ProjectionExpr {
+ expr: Arc::new(Column::new("b", 1)),
+ alias: "y".to_string(),
+ },
+ ],
+ input,
+ )?;
+
+ // filter "x > 5"
+ let filter1 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("x", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ // filter "y < 10"
+ let filter2 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("y", 1)),
+ Operator::Lt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter1, filter2],
+ &ConfigOptions::default(),
+ )?;
+
+ // Should be converted to "a > 5" and "b < 10"
+ let expected_filter1 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let expected_filter2 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 1)),
+ Operator::Lt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(pushed_filters.len(), 2);
+ // Note: The order of filters is preserved
+ assert_eq!(
+ format!("{}", pushed_filters[0].predicate),
+ format!("{}", expected_filter1)
+ );
+ assert_eq!(
+ format!("{}", pushed_filters[1].predicate),
+ format!("{}", expected_filter2)
+ );
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_mixed_columns() -> Result<()> {
+ let input_schema = Schema::new(vec![
+ Field::new("a", DataType::Int32, false),
+ Field::new("b", DataType::Int32, false),
+ ]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a" as "x", "b" as "b" (pass through)
+ let projection = ProjectionExec::try_new(
+ vec![
+ ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "x".to_string(),
+ },
+ ProjectionExpr {
+ expr: Arc::new(Column::new("b", 1)),
+ alias: "b".to_string(),
+ },
+ ],
+ input,
+ )?;
+
+ // filter "x > 5"
+ let filter1 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("x", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ // filter "b < 10" (using output index 1 which corresponds to 'b')
+ let filter2 = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 1)),
+ Operator::Lt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter1, filter2],
+ &ConfigOptions::default(),
+ )?;
+
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(pushed_filters.len(), 2);
+ // "x" -> "a" (index 0)
+ let expected_filter1 = "a@0 > 5";
+ // "b" -> "b" (index 1)
+ let expected_filter2 = "b@1 < 10";
+
+ assert_eq!(format!("{}", pushed_filters[0].predicate),
expected_filter1);
+ assert_eq!(format!("{}", pushed_filters[1].predicate),
expected_filter2);
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_complex_expression() -> Result<()> {
+ let input_schema = Schema::new(vec![Field::new("a", DataType::Int32,
false)]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a + 1" as "z"
+ let projection = ProjectionExec::try_new(
+ vec![ProjectionExpr {
+ expr: Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Plus,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(1)))),
+ )),
+ alias: "z".to_string(),
+ }],
+ input,
+ )?;
+
+ // filter "z > 10"
+ let filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("z", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(10)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter],
+ &ConfigOptions::default(),
+ )?;
+
+ // expand to `a + 1 > 10`
+ let pushed_filters = &description.parent_filters()[0];
+ assert!(matches!(pushed_filters[0].discriminant, PushedDown::Yes));
+ assert_eq!(format!("{}", pushed_filters[0].predicate), "a@0 + 1 > 10");
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_unknown_column() -> Result<()> {
+ let input_schema = Schema::new(vec![Field::new("a", DataType::Int32,
false)]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a" as "a"
+ let projection = ProjectionExec::try_new(
+ vec![ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "a".to_string(),
+ }],
+ input,
+ )?;
+
+ // filter "unknown_col > 5" - using a column name that doesn't exist
in projection output
+ // Column constructor: name, index. Index 1 doesn't exist.
+ let filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("unknown_col", 1)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter],
+ &ConfigOptions::default(),
+ )?;
+
+ let pushed_filters = &description.parent_filters()[0];
+ assert!(matches!(pushed_filters[0].discriminant, PushedDown::No));
+ // The column shouldn't be found in the alias map, so it should become
UnKnownColumn
+ assert_eq!(
+ format!("{}", pushed_filters[0].predicate),
+ "unknown_col > 5"
+ );
+
+ Ok(())
+ }
+
+ /// Test that `DynamicFilterPhysicalExpr` can correctly update its child
expression
Review Comment:
I feel this is probably better off as a combination of unit tests on
`DynamicFilterPhysicalExpr` (which may already exist) + integration tests in
datafusion/core/tests/parquet/filter_pushdown.rs + SLT integration tests
##########
datafusion/physical-plan/src/projection.rs:
##########
@@ -1270,4 +1302,357 @@ mod tests {
);
assert!(stats.total_byte_size.is_exact().unwrap_or(false));
}
+
+ #[test]
+ fn test_filter_pushdown_with_alias() -> Result<()> {
+ let input_schema = Schema::new(vec![Field::new("a", DataType::Int32,
false)]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics::new_unknown(&input_schema),
+ input_schema.clone(),
+ ));
+
+ // project "a" as "b"
+ let projection = ProjectionExec::try_new(
+ vec![ProjectionExpr {
+ expr: Arc::new(Column::new("a", 0)),
+ alias: "b".to_string(),
+ }],
+ input,
+ )?;
+
+ // filter "b > 5"
+ let filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("b", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ let description = projection.gather_filters_for_pushdown(
+ FilterPushdownPhase::Post,
+ vec![filter],
+ &ConfigOptions::default(),
+ )?;
+
+ // Should be converted to "a > 5"
+ // "a" is index 0 in input
+ let expected_filter = Arc::new(BinaryExpr::new(
+ Arc::new(Column::new("a", 0)),
+ Operator::Gt,
+ Arc::new(Literal::new(ScalarValue::Int32(Some(5)))),
+ )) as Arc<dyn PhysicalExpr>;
+
+ assert_eq!(description.self_filters(), vec![vec![]]);
+ let pushed_filters = &description.parent_filters()[0];
+ assert_eq!(
+ format!("{}", pushed_filters[0].predicate),
+ format!("{}", expected_filter)
+ );
+
+ Ok(())
+ }
+
+ #[test]
+ fn test_filter_pushdown_with_multiple_aliases() -> Result<()> {
+ let input_schema = Schema::new(vec![
+ Field::new("a", DataType::Int32, false),
+ Field::new("b", DataType::Int32, false),
+ ]);
+ let input = Arc::new(StatisticsExec::new(
+ Statistics {
+ column_statistics: vec![Default::default();
input_schema.fields().len()],
+ ..Default::default()
+ },
+ input_schema.clone(),
+ ));
+
+ // project "a" as "x", "b" as "y"
Review Comment:
Can we add a test where we do `a as b, b as a`?
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