add better descriptions of logical operator fields

Author: connortsui20

optd-core/src/operator/relational/logical/filter.rs

@@ -8,5 +8,8 @@ pub struct Filter<Relation, Scalar> {
     /// The input relation.
     pub child: Relation,
     /// The filter expression denoting the predicate condition for this filter operation.
+    ///
+    /// For example, a filter predicate could be `column_a > 42`, or it could be something like
+    /// `column_b < 100 AND column_c > 1000`.
     pub predicate: Scalar,
 }

optd-core/src/operator/relational/logical/join.rs

@@ -13,5 +13,7 @@ pub struct Join<Relation, Scalar> {
     /// The right input relation.
     pub right: Relation,
     /// The join expression denoting the join condition that links the two input relations.
+    ///
+    /// For example, a join operation could have a condition on `t1.id = t2.id` (an equijoin).
     pub condition: Scalar,
 }

optd-core/src/operator/relational/logical/scan.rs

@@ -9,5 +9,8 @@ pub struct Scan<Scalar> {
     /// TODO(alexis) Mocked for now.
     pub table_name: String,
     /// An optional filter expression for predicate pushdown into scan operators.
+    ///
+    /// For example, a `Filter(Scan(A), column_a < 42)` can be converted into a predicate pushdown
+    /// `Scan(A, column < 42)` to prevent having to materialize many tuples.
     pub predicate: Option<Scalar>,
 }