Fix issue with EXPLAIN and parallel local queries

We need to be sure to pass CURSOR_OPT_PARALLEL in 9.6 for any SELECT
query without an INTO clause.

src/backend/distributed/planner/multi_explain.c

@@ -18,6 +18,7 @@
 #include "commands/dbcommands.h"
 #include "commands/explain.h"
 #include "commands/tablecmds.h"
+#include "optimizer/cost.h"
 #include "distributed/citus_nodefuncs.h"
 #include "distributed/multi_client_executor.h"
 #include "distributed/multi_executor.h"
@@ -108,6 +109,11 @@ MultiExplainOneQuery(Query *query, IntoClause *into, ExplainState *es,
 	instr_time planDuration;
 	Query *originalQuery = NULL;
 	RelationRestrictionContext *restrictionContext = NULL;
+#if PG_VERSION_NUM >= 90600
+	int cursorOptions = into ? 0 : CURSOR_OPT_PARALLEL_OK;
+#else
+	int cursorOptions = 0;
+#endif
 
 	/* if local query, run the standard explain and return */
 	bool localQuery = !NeedsDistributedPlanning(query);
@@ -118,7 +124,7 @@ MultiExplainOneQuery(Query *query, IntoClause *into, ExplainState *es,
 		INSTR_TIME_SET_CURRENT(planStart);
 
 		/* plan the query */
-		plan = pg_plan_query(query, 0, params);
+		plan = pg_plan_query(query, cursorOptions, params);
 
 		INSTR_TIME_SET_CURRENT(planDuration);
 		INSTR_TIME_SUBTRACT(planDuration, planStart);
@@ -143,7 +149,7 @@ MultiExplainOneQuery(Query *query, IntoClause *into, ExplainState *es,
 	PG_TRY();
 	{
 		/* call standard planner to modify the query structure before multi planning */
-		initialPlan = standard_planner(query, 0, params);
+		initialPlan = standard_planner(query, cursorOptions, params);
 
 		commandType = initialPlan->commandType;
 		if (commandType == CMD_INSERT || commandType == CMD_UPDATE ||

src/test/regress/expected/multi_explain.out

@@ -645,3 +645,29 @@ EXPLAIN (COSTS FALSE, FORMAT YAML)
             Parallel Aware: false
             Relation Name: "pg_merge_job_570036"
             Alias: "pg_merge_job_570036"
+-- test parallel aggregates
+SET parallel_setup_cost=0;
+SET parallel_tuple_cost=0;
+SET min_parallel_relation_size=0;
+SET max_parallel_workers_per_gather=4;
+-- ensure local plans display correctly
+CREATE TABLE lineitem_clone (LIKE lineitem);
+EXPLAIN (COSTS FALSE) SELECT avg(l_linenumber) FROM lineitem_clone;
+Finalize Aggregate
+  ->  Gather
+        Workers Planned: 3
+        ->  Partial Aggregate
+              ->  Parallel Seq Scan on lineitem_clone
+-- ensure distributed plans don't break
+EXPLAIN (COSTS FALSE) SELECT avg(l_linenumber) FROM lineitem;
+Distributed Query into pg_merge_job_570037
+  Executor: Task-Tracker
+  Task Count: 8
+  Tasks Shown: One of 8
+  ->  Task
+        Node: host=localhost port=57637 dbname=regression
+        ->  Aggregate
+              ->  Seq Scan on lineitem_290001 lineitem
+Master Query
+  ->  Aggregate
+        ->  Seq Scan on pg_merge_job_570037

src/test/regress/expected/multi_explain_0.out

@@ -615,3 +615,30 @@ EXPLAIN (COSTS FALSE, FORMAT YAML)
             Parent Relationship: "Outer"
             Relation Name: "pg_merge_job_570036"
             Alias: "pg_merge_job_570036"
+-- test parallel aggregates
+SET parallel_setup_cost=0;
+ERROR:  unrecognized configuration parameter "parallel_setup_cost"
+SET parallel_tuple_cost=0;
+ERROR:  unrecognized configuration parameter "parallel_tuple_cost"
+SET min_parallel_relation_size=0;
+ERROR:  unrecognized configuration parameter "min_parallel_relation_size"
+SET max_parallel_workers_per_gather=4;
+ERROR:  unrecognized configuration parameter "max_parallel_workers_per_gather"
+-- ensure local plans display correctly
+CREATE TABLE lineitem_clone (LIKE lineitem);
+EXPLAIN (COSTS FALSE) SELECT avg(l_linenumber) FROM lineitem_clone;
+Aggregate
+  ->  Seq Scan on lineitem_clone
+-- ensure distributed plans don't break
+EXPLAIN (COSTS FALSE) SELECT avg(l_linenumber) FROM lineitem;
+Distributed Query into pg_merge_job_570037
+  Executor: Task-Tracker
+  Task Count: 8
+  Tasks Shown: One of 8
+  ->  Task
+        Node: host=localhost port=57637 dbname=regression
+        ->  Aggregate
+              ->  Seq Scan on lineitem_290001 lineitem
+Master Query
+  ->  Aggregate
+        ->  Seq Scan on pg_merge_job_570037

src/test/regress/sql/multi_explain.sql

@@ -188,3 +188,16 @@ EXPLAIN (COSTS FALSE, FORMAT YAML)
 	WHERE l_orderkey = o_orderkey
 	AND o_custkey = c_custkey
 	AND l_suppkey = s_suppkey;
+
+-- test parallel aggregates
+SET parallel_setup_cost=0;
+SET parallel_tuple_cost=0;
+SET min_parallel_relation_size=0;
+SET max_parallel_workers_per_gather=4;
+
+-- ensure local plans display correctly
+CREATE TABLE lineitem_clone (LIKE lineitem);
+EXPLAIN (COSTS FALSE) SELECT avg(l_linenumber) FROM lineitem_clone;
+
+-- ensure distributed plans don't break
+EXPLAIN (COSTS FALSE) SELECT avg(l_linenumber) FROM lineitem;