Introduce a new GUC force_parallel_mode for testing purposes.

When force_parallel_mode = true, we enable the parallel mode restrictions
for all queries for which this is believed to be safe.  For the subset of
those queries believed to be safe to run entirely within a worker, we spin
up a worker and run the query there instead of running it in the
original process.  When force_parallel_mode = regress, make additional
changes to allow the regression tests to run cleanly even though parallel
workers have been injected under the hood.

Taken together, this facilitates both better user testing and better
regression testing of the parallelism code.

Robert Haas, with help from Amit Kapila and Rushabh Lathia.

1 files changed, 55 insertions, 18 deletions

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index a09b4b5b479..a3cc27464c1 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -48,10 +48,12 @@
 #include "storage/dsm_impl.h"
 #include "utils/rel.h"
 #include "utils/selfuncs.h"
+#include "utils/syscache.h"
 
 
-/* GUC parameter */
+/* GUC parameters */
 double		cursor_tuple_fraction = DEFAULT_CURSOR_TUPLE_FRACTION;
+int			force_parallel_mode = FORCE_PARALLEL_OFF;
 
 /* Hook for plugins to get control in planner() */
 planner_hook_type planner_hook = NULL;
@@ -230,25 +232,31 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 		!has_parallel_hazard((Node *) parse, true);
 
 	/*
-	 * glob->parallelModeOK should tell us whether it's necessary to impose
-	 * the parallel mode restrictions, but we don't actually want to impose
-	 * them unless we choose a parallel plan, so that people who mislabel
-	 * their functions but don't use parallelism anyway aren't harmed.
-	 * However, it's useful for testing purposes to be able to force the
-	 * restrictions to be imposed whenever a parallel plan is actually chosen
-	 * or not.
+	 * glob->parallelModeNeeded should tell us whether it's necessary to
+	 * impose the parallel mode restrictions, but we don't actually want to
+	 * impose them unless we choose a parallel plan, so that people who
+	 * mislabel their functions but don't use parallelism anyway aren't
+	 * harmed. But when force_parallel_mode is set, we enable the restrictions
+	 * whenever possible for testing purposes.
 	 *
-	 * (It's been suggested that we should always impose these restrictions
-	 * whenever glob->parallelModeOK is true, so that it's easier to notice
-	 * incorrectly-labeled functions sooner.  That might be the right thing to
-	 * do, but for now I've taken this approach.  We could also control this
-	 * with a GUC.)
+	 * glob->wholePlanParallelSafe should tell us whether it's OK to stick a
+	 * Gather node on top of the entire plan.  However, it only needs to be
+	 * accurate when force_parallel_mode is 'on' or 'regress', so we don't
+	 * bother doing the work otherwise.  The value we set here is just a
+	 * preliminary guess; it may get changed from true to false later, but
+	 * not visca versa.
 	 */
-#ifdef FORCE_PARALLEL_MODE
-	glob->parallelModeNeeded = glob->parallelModeOK;
-#else
-	glob->parallelModeNeeded = false;
-#endif
+	if (force_parallel_mode == FORCE_PARALLEL_OFF || !glob->parallelModeOK)
+	{
+		glob->parallelModeNeeded = false;
+		glob->wholePlanParallelSafe = false;	/* either false or don't care */
+	}
+	else
+	{
+		glob->parallelModeNeeded = true;
+		glob->wholePlanParallelSafe =
+			!has_parallel_hazard((Node *) parse, false);
+	}
 
 	/* Determine what fraction of the plan is likely to be scanned */
 	if (cursorOptions & CURSOR_OPT_FAST_PLAN)
@@ -293,6 +301,35 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 	}
 
 	/*
+	 * At present, we don't copy subplans to workers.  The presence of a
+	 * subplan in one part of the plan doesn't preclude the use of parallelism
+	 * in some other part of the plan, but it does preclude the possibility of
+	 * regarding the entire plan parallel-safe.
+	 */
+	if (glob->subplans != NULL)
+		glob->wholePlanParallelSafe = false;
+
+	/*
+	 * Optionally add a Gather node for testing purposes, provided this is
+	 * actually a safe thing to do.
+	 */
+	if (glob->wholePlanParallelSafe &&
+		force_parallel_mode != FORCE_PARALLEL_OFF)
+	{
+		Gather	   *gather = makeNode(Gather);
+
+		gather->plan.targetlist = top_plan->targetlist;
+		gather->plan.qual = NIL;
+		gather->plan.lefttree = top_plan;
+		gather->plan.righttree = NULL;
+		gather->num_workers = 1;
+		gather->single_copy = true;
+		gather->invisible = (force_parallel_mode == FORCE_PARALLEL_REGRESS);
+		root->glob->parallelModeNeeded = true;
+		top_plan = &gather->plan;
+	}
+
+	/*
 	 * If any Params were generated, run through the plan tree and compute
 	 * each plan node's extParam/allParam sets.  Ideally we'd merge this into
 	 * set_plan_references' tree traversal, but for now it has to be separate