add get method for id pred and add comments

Author: xx01cyx

optd-cost-model/src/common/predicates/attr_ref_pred.rs

@@ -7,6 +7,10 @@ use super::id_pred::IdPred;
 /// An [`AttributeRefPred`] has two children:
 /// 1. The table id, represented by an [`IdPred`].
 /// 2. The index of the column, represented by an [`IdPred`].
+///
+/// Currently, [`AttributeRefPred`] only holds base table attributes, i.e. attributes
+/// that already exist in the table. More complex structures may be introduced in the
+/// future to represent derived attributes (e.g. t.v1 + t.v2).
 #[derive(Clone, Debug)]
 pub struct AttributeRefPred(pub ArcPredicateNode);
 

optd-cost-model/src/common/predicates/id_pred.rs

@@ -23,6 +23,11 @@ impl IdPred {
             .into(),
         )
     }
+
+    /// Gets the id stored in the predicate.
+    pub fn id(&self) -> usize {
+        self.0.data.clone().unwrap().as_u64() as usize
+    }
 }
 
 impl ReprPredicateNode for IdPred {