Merge pull request #330 from noahisaksen/fix-limit-during-parallel-execution

Fix #328 - Dont do limit pushdown during parallel execution

Author: Mytherin

src/storage/postgres_optimizer.cpp

@@ -53,6 +53,11 @@ static void OptimizePostgresScanLimitPushdown(unique_ptr<LogicalOperator> &op) {
 		}
 
 		auto &bind_data = get.bind_data->Cast<PostgresBindData>();
+		if (bind_data.max_threads != 1 || !bind_data.can_use_main_thread) {
+			// cannot push down limit/offset if we are not using the main thread
+			OptimizePostgresScanLimitPushdown(op->children[0]);
+			return;
+		}
 
 		string generated_limit_clause = "";
 		if (limit.limit_val.Type() != LimitNodeType::UNSET) {

test/sql/storage/limit.test

@@ -39,3 +39,35 @@ query II
 EXPLAIN FROM s.large_tbl LIMIT 5;
 ----
 logical_opt	<!REGEX>:.*LIMIT.*
+
+
+statement ok
+set pg_pages_per_task=1
+
+query I
+FROM s.large_tbl LIMIT 5
+----
+0
+1
+2
+3
+4
+
+query I
+FROM s.large_tbl LIMIT 5 OFFSET 5
+----
+5
+6
+7
+8
+9
+
+statement ok
+set explain_output='optimized_only'
+
+# limit is still in plan as we were not able to push down due to parallel execution
+
+query II
+EXPLAIN FROM s.large_tbl LIMIT 5;
+----
+logical_opt	<REGEX>:.*LIMIT.*