Fix EXPLAIN ANALYZE SELECT INTO not to choose a parallel plan.

We don't support any parallel write operations at present, so choosing
a parallel plan causes us to error out.  Also, add a new regression
test that uses EXPLAIN ANALYZE SELECT INTO; if we'd had this previously,
force_parallel_mode testing would have caught this issue.

Mithun Cy and Robert Haas

src/backend/commands/explain.c

@@ -347,7 +347,7 @@ ExplainOneQuery(Query *query, IntoClause *into, ExplainState *es,
 		INSTR_TIME_SET_CURRENT(planstart);
 
 		/* plan the query */
-		plan = pg_plan_query(query, CURSOR_OPT_PARALLEL_OK, params);
+		plan = pg_plan_query(query, into ? 0 : CURSOR_OPT_PARALLEL_OK, params);
 
 		INSTR_TIME_SET_CURRENT(planduration);
 		INSTR_TIME_SUBTRACT(planduration, planstart);

src/test/regress/expected/select_into.out

@@ -74,7 +74,14 @@ SELECT * FROM created_table;
  4567890123456789 | -4567890123456789
 (5 rows)
 
+-- Try EXPLAIN ANALYZE SELECT INTO, but hide the output since it won't
+-- be stable.
+DO $$
+BEGIN
+	EXECUTE 'EXPLAIN ANALYZE SELECT * INTO TABLE easi FROM int8_tbl';
+END$$;
 DROP TABLE created_table;
+DROP TABLE easi;
 --
 -- Disallowed uses of SELECT ... INTO.  All should fail
 --

src/test/regress/sql/select_into.sql

@@ -66,7 +66,15 @@ SELECT make_table();
 
 SELECT * FROM created_table;
 
+-- Try EXPLAIN ANALYZE SELECT INTO, but hide the output since it won't
+-- be stable.
+DO $$
+BEGIN
+	EXECUTE 'EXPLAIN ANALYZE SELECT * INTO TABLE easi FROM int8_tbl';
+END$$;
+
 DROP TABLE created_table;
+DROP TABLE easi;
 
 --
 -- Disallowed uses of SELECT ... INTO.  All should fail