alamb commented on code in PR #12590:
URL: https://github.com/apache/datafusion/pull/12590#discussion_r1771415270
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datafusion/physical-expr/src/equivalence/properties.rs:
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@@ -77,11 +87,39 @@ use itertools::Itertools;
/// └---┴---┘
/// ```
///
-/// where columns `a` and `b` always have the same value. We keep track of such
-/// equivalences inside this object. With this information, we can optimize
-/// things like partitioning. For example, if the partition requirement is
-/// `Hash(a)` and output partitioning is `Hash(b)`, then we can deduce that
-/// the existing partitioning satisfies the requirement.
+/// In this case, columns `a` and `b` always have the same value, which can of
+/// such equivalences inside this object. With this information, Datafusion can
+/// optimize operations such as. For example, if the partition requirement is
+/// `Hash(a)` and output partitioning is `Hash(b)`, then DataFusion avoids
+/// repartitioning the data as the existing partitioning satisfies the
+/// requirement.
+///
+/// # Code Example
+/// ```
+/// # use std::sync::Arc;
+/// # use arrow_schema::{Schema, Field, DataType, SchemaRef};
+/// # use datafusion_physical_expr::{ConstExpr, EquivalenceProperties};
+/// # use datafusion_physical_expr::expressions::col;
+/// use datafusion_physical_expr_common::sort_expr::PhysicalSortExpr;
+/// # let schema: SchemaRef = Arc::new(Schema::new(vec![
+/// # Field::new("a", DataType::Int32, false),
+/// # Field::new("b", DataType::Int32, false),
+/// # Field::new("c", DataType::Int32, false),
+/// # ]));
+/// # let col_a = col("a", &schema).unwrap();
+/// # let col_b = col("b", &schema).unwrap();
+/// # let col_c = col("c", &schema).unwrap();
+/// // This object represents data that is sorted by a ASC, c DESC
+/// // with a single constant value of b
+/// let mut eq_properties = EquivalenceProperties::new(schema)
+/// .with_constants(vec![ConstExpr::from(col_b)]);
+/// eq_properties.add_new_ordering(vec![
+/// PhysicalSortExpr::new_default(col_a).asc(),
+/// PhysicalSortExpr::new_default(col_c).desc(),
+/// ]);
+///
+/// assert_eq!(eq_properties.to_string(), "order: [a@0 ASC,c@2 DESC], const:
[b@1]")
Review Comment:
This shows the example of the Display property I wanted -- that shows the
columns in a reasonably human friendly way
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