perf: simplify and remove allocations in ref resolution

src/executor/engine/cartridge/backend/sql/ir.rs

@@ -168,15 +168,8 @@ impl ExecutionPlan {
                                 })?;
                             }
                             Expression::Reference { position, .. } => {
-                                let rel_id: usize = ir_plan
-                                    .get_relational_from_reference_node(*id)?
-                                    .into_iter()
-                                    .next()
-                                    .ok_or_else(|| {
-                                        QueryPlannerError::CustomError(
-                                            "Reference points to a non-relational node.".into(),
-                                        )
-                                    })?;
+                                let rel_id: usize =
+                                    ir_plan.get_relational_from_reference_node(*id)?;
                                 let rel_node = ir_plan.get_relation_node(rel_id)?;
                                 let alias = &ir_plan.get_alias_from_reference_node(expr)?;
 

src/ir/explain.rs

@@ -47,15 +47,7 @@ impl Col {
                     )));
                 }
                 Expression::Reference { position, .. } => {
-                    let rel_id: usize = plan
-                        .get_relational_from_reference_node(*id)?
-                        .into_iter()
-                        .next()
-                        .ok_or_else(|| {
-                            QueryPlannerError::CustomError(
-                                "Reference doesn't have child relation node".into(),
-                            )
-                        })?;
+                    let rel_id: usize = plan.get_relational_from_reference_node(*id)?;
 
                     let rel_node = plan.get_relation_node(rel_id)?;
                     let alias = plan.get_alias_from_reference_node(current_node)?;
@@ -236,15 +228,7 @@ impl Row {
                     )));
                 }
                 Expression::Reference { position, .. } => {
-                    let rel_id: usize = plan
-                        .get_relational_from_reference_node(*child)?
-                        .into_iter()
-                        .next()
-                        .ok_or_else(|| {
-                            QueryPlannerError::CustomError(
-                                "Reference doesn't have a child relation node.".into(),
-                            )
-                        })?;
+                    let rel_id: usize = plan.get_relational_from_reference_node(*child)?;
 
                     let rel_node = plan.get_relation_node(rel_id)?;
 

src/ir/expression.rs

@@ -682,7 +682,6 @@ impl Plan {
     }
 
     /// A list of relational nodes that makes up the reference.
-    /// For references in the scan node
     ///
     /// # Errors
     /// - reference is invalid
@@ -690,7 +689,7 @@ impl Plan {
     pub fn get_relational_from_reference_node(
         &self,
         ref_id: usize,
-    ) -> Result<HashSet<usize, RandomState>, QueryPlannerError> {
+    ) -> Result<usize, QueryPlannerError> {
         if let Node::Expression(Expression::Reference {
             targets, parent, ..
         }) = self.get_node(ref_id)?
@@ -700,20 +699,30 @@ impl Plan {
             ))?;
             let rel = self.get_relation_node(referred_rel_id)?;
             if let Relational::Insert { .. } = rel {
-                let mut rel_nodes: HashSet<usize, RandomState> =
-                    HashSet::with_capacity_and_hasher(1, RandomState::new());
-                rel_nodes.insert(referred_rel_id);
-                return Ok(rel_nodes);
+                return Ok(referred_rel_id);
             } else if let Some(children) = rel.children() {
-                if let Some(positions) = targets {
-                    let mut rel_nodes: HashSet<usize, RandomState> =
-                        HashSet::with_capacity_and_hasher(positions.len(), RandomState::new());
-                    for pos in positions {
-                        if let Some(child) = children.get(*pos) {
-                            rel_nodes.insert(*child);
-                        }
+                match targets {
+                    None => {
+                        return Err(QueryPlannerError::CustomError(
+                            "Reference node has no targets".into(),
+                        ))
                     }
-                    return Ok(rel_nodes);
+                    Some(positions) => match (positions.get(0), positions.get(1)) {
+                        (Some(first), None) => {
+                            let child_id = *children.get(*first).ok_or_else(|| {
+                                QueryPlannerError::CustomError(
+                                    "Relational node has no child at first position".into(),
+                                )
+                            })?;
+                            return Ok(child_id);
+                        }
+                        _ => {
+                            return Err(QueryPlannerError::CustomError(
+                                "Reference expected to point exactly a single relational node"
+                                    .into(),
+                            ))
+                        }
+                    },
                 }
             }
         }

src/ir/operator.rs

@@ -410,12 +410,8 @@ impl Relational {
                 if let Expression::Alias { child, .. } = col_node {
                     let child_node = plan.get_expression_node(*child)?;
                     if let Expression::Reference { position: pos, .. } = child_node {
-                        let rel_ids = plan.get_relational_from_reference_node(*child)?;
-                        let rel_node = plan.get_relation_node(
-                            rel_ids.into_iter().next().ok_or_else(|| {
-                                QueryPlannerError::CustomError(String::from("No relational node"))
-                            })?,
-                        )?;
+                        let rel_id = plan.get_relational_from_reference_node(*child)?;
+                        let rel_node = plan.get_relation_node(rel_id)?;
                         return rel_node.scan_name(plan, *pos);
                     }
                 } else {