fix(cubesql): Correctly calculate eclass cost for recursive nodes (#9947)

Author: MazterQyou

rust/cubesql/cubesql/src/compile/mod.rs

@@ -17655,4 +17655,47 @@ LIMIT {{ limit }}{% endif %}"#.to_string(),
         let sql = logical_plan.find_cube_scan_wrapped_sql().wrapped_sql.sql;
         assert!(sql.contains("TRUNCATE(EXTRACT(month FROM "));
     }
+
+    #[tokio::test]
+    async fn test_top_down_extractor_cache() {
+        if !Rewriter::sql_push_down_enabled() {
+            return;
+        }
+        init_testing_logger();
+
+        let logical_plan = convert_select_to_query_plan(
+            r#"
+            SELECT
+                id::integer AS id,
+                customer_gender
+            FROM KibanaSampleDataEcommerce
+            WHERE id = 5
+            "#
+            .to_string(),
+            DatabaseProtocol::PostgreSQL,
+        )
+        .await
+        .as_logical_plan();
+
+        assert_eq!(
+            logical_plan.find_cube_scan().request,
+            V1LoadRequestQuery {
+                measures: Some(vec![]),
+                dimensions: Some(vec![
+                    "KibanaSampleDataEcommerce.id".to_string(),
+                    "KibanaSampleDataEcommerce.customer_gender".to_string(),
+                ]),
+                segments: Some(vec![]),
+                order: Some(vec![]),
+                filters: Some(vec![V1LoadRequestQueryFilterItem {
+                    member: Some("KibanaSampleDataEcommerce.id".to_string()),
+                    operator: Some("equals".to_string()),
+                    values: Some(vec!["5".to_string()]),
+                    ..Default::default()
+                }]),
+                ungrouped: Some(true),
+                ..Default::default()
+            }
+        )
+    }
 }

rust/cubesql/cubesql/src/compile/rewrite/cost.rs

@@ -1,4 +1,6 @@
-use std::{collections::HashMap, fmt::Debug, hash::Hash, marker::PhantomData, sync::Arc};
+use std::{
+    collections::HashMap, fmt::Debug, hash::Hash, marker::PhantomData, mem::take, sync::Arc,
+};
 
 use crate::{
     compile::rewrite::{
@@ -531,6 +533,9 @@ where
     egraph: &'a EGraph<L, A>,
     // Caches results. `None` for nodes in progress to prevent recursion
     extract_map: HashMap<IdWithState<L, S>, Option<(usize, C)>>,
+    has_deep_recursion: bool,
+    // Cache for second pass to calculate recursive nodes cost.
+    extract_map_recursive_cache: Option<HashMap<IdWithState<L, S>, Option<(usize, C)>>>,
     cost_fn: Arc<CF>,
     root_state: Arc<S>,
 }
@@ -547,6 +552,8 @@ where
         Self {
             egraph,
             extract_map: HashMap::new(),
+            has_deep_recursion: false,
+            extract_map_recursive_cache: None,
             cost_fn: Arc::new(cost_fn),
             root_state: Arc::new(root_state),
         }
@@ -555,8 +562,17 @@ where
     /// Returns cost and path for best plan for provided root eclass.
     ///
     /// If all nodes happen to be recursive, returns `None`.
+    ///
+    /// If there were any nodes with deep recursion, the cost is calculated in two passes;
+    /// the second pass fetches the cost from the extract map obtained on the first pass
+    /// for recursive nodes only.
     pub fn find_best(&mut self, root: Id) -> Option<(C, RecExpr<L>)> {
-        let cost = self.extract(root, Arc::clone(&self.root_state))?;
+        let mut cost = self.extract(root, Arc::clone(&self.root_state))?;
+        if self.has_deep_recursion {
+            self.extract_map_recursive_cache = Some(take(&mut self.extract_map));
+            cost = self.extract(root, Arc::clone(&self.root_state))?;
+        }
+
         let root_id_with_state = IdWithState::new(root, Arc::clone(&self.root_state));
         let root_node = self.choose_node(&root_id_with_state)?;
         let recexpr =
@@ -569,14 +585,33 @@ where
     /// Recursively extracts the cost of each node in the eclass
     /// and returns cost of the node with least cost based on the passed state,
     /// caching the cost together with node index inside eclass in `extract_map`.
+    /// If `extract_map_recursive_cache` is available, fetches the costs
+    /// of deep recursion nodes from there.
     ///
     /// Yields `None` if eclass is already in progress
     /// or all its nodes happen to be recursive.
     fn extract(&mut self, eclass: Id, state: Arc<S>) -> Option<C> {
         let id_with_state = IdWithState::new(eclass, state);
         if let Some(cached_index_and_cost) = self.extract_map.get(&id_with_state) {
-            // TODO: avoid cloning here?
-            return cached_index_and_cost.as_ref().map(|(_, cost)| cost.clone());
+            // If the cost has been computed, return it
+            if let Some((_, cached_cost)) = cached_index_and_cost {
+                // TODO: avoid cloning here?
+                return Some(cached_cost.clone());
+            }
+
+            // If the cost is recursive, fetch from recursive cache if available
+            if let Some(extract_map_recursive_cache) = &self.extract_map_recursive_cache {
+                if let Some(Some((_, cached_cost))) =
+                    extract_map_recursive_cache.get(&id_with_state)
+                {
+                    // TODO: avoid cloning here?
+                    return Some(cached_cost.clone());
+                }
+            }
+
+            // Otherwise, mark this extractor as having deep recursion
+            self.has_deep_recursion = true;
+            return None;
         }
 
         // Mark this eclass as in progress
@@ -595,6 +630,12 @@ where
             // Recursively get children cost
             let mut total_node_cost = this_node_cost;
             for child in node.children() {
+                // If a child is recursive to self, skip this node, as it will never compute
+                // the cost
+                if child == &eclass {
+                    continue 'nodes;
+                }
+
                 let Some(child_cost) = self.extract(*child, Arc::clone(&new_state)) else {
                     // This path is inevitably recursive, try the next node
                     continue 'nodes;