fix(cubesql): Pushdown LIMIT 0 to CubeScan

Author: mcheshkov

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

@@ -851,13 +851,69 @@ mod tests {
                 time_dimensions: None,
                 order: Some(vec![]),
                 limit: Some(0),
-                offset: None,
+                offset: Some(0),
                 filters: None,
                 ungrouped: Some(true),
             }
         );
     }
 
+    #[tokio::test]
+    async fn test_pushdown_limit_0_grouped() {
+        if !Rewriter::sql_push_down_enabled() {
+            return;
+        }
+        init_testing_logger();
+
+        let logical_plan = convert_select_to_query_plan(
+            "SELECT KibanaSampleDataEcommerce.customer_gender FROM \"public\".\"KibanaSampleDataEcommerce\" GROUP BY 1 LIMIT 0".to_string(),
+            DatabaseProtocol::PostgreSQL
+        ).await.as_logical_plan();
+
+        assert_eq!(
+            logical_plan.find_cube_scan().request,
+            V1LoadRequestQuery {
+                measures: Some(vec![]),
+                dimensions: Some(vec!["KibanaSampleDataEcommerce.customer_gender".to_string()]),
+                segments: Some(vec![]),
+                time_dimensions: None,
+                order: Some(vec![]),
+                limit: Some(0),
+                offset: None,
+                filters: None,
+                ungrouped: None,
+            }
+        )
+    }
+
+    #[tokio::test]
+    async fn test_pushdown_limit_0_ungrouped() {
+        if !Rewriter::sql_push_down_enabled() {
+            return;
+        }
+        init_testing_logger();
+
+        let logical_plan = convert_select_to_query_plan(
+            "SELECT KibanaSampleDataEcommerce.customer_gender FROM \"public\".\"KibanaSampleDataEcommerce\" LIMIT 0".to_string(),
+            DatabaseProtocol::PostgreSQL
+        ).await.as_logical_plan();
+
+        assert_eq!(
+            logical_plan.find_cube_scan().request,
+            V1LoadRequestQuery {
+                measures: Some(vec![]),
+                dimensions: Some(vec!["KibanaSampleDataEcommerce.customer_gender".to_string()]),
+                segments: Some(vec![]),
+                time_dimensions: None,
+                order: Some(vec![]),
+                limit: Some(0),
+                offset: None,
+                filters: None,
+                ungrouped: Some(true),
+            }
+        )
+    }
+
     #[tokio::test]
     async fn tableau_projection_with_casts() {
         if !Rewriter::sql_push_down_enabled() {

rust/cubesql/cubesql/src/compile/rewrite/rules/members.rs

@@ -1573,14 +1573,14 @@ impl MemberRules {
         move |egraph, subst| {
             let mut skip_value = None;
             for skip in var_iter!(egraph[subst[skip_var]], LimitSkip) {
-                if skip.unwrap_or_default() > 0 {
+                if skip.is_some() {
                     skip_value = *skip;
                     break;
                 }
             }
             let mut fetch_value = None;
             for fetch in var_iter!(egraph[subst[fetch_var]], LimitFetch) {
-                if fetch.unwrap_or_default() > 0 {
+                if fetch.is_some() {
                     fetch_value = *fetch;
                     break;
                 }