Pass InitPlan values to workers via Gather (Merge).

If a PARAM_EXEC parameter is used below a Gather (Merge) but the InitPlan
that computes it is attached to or above the Gather (Merge), force the
value to be computed before starting parallelism and pass it down to all
workers.  This allows us to use parallelism in cases where it previously
would have had to be rejected as unsafe.  We do - in this case - lose the
optimization that the value is only computed if it's actually used.  An
alternative strategy would be to have the first worker that needs the value
compute it, but one downside of that approach is that we'd then need to
select a parallel-safe path to compute the parameter value; it couldn't for
example contain a Gather (Merge) node.  At some point in the future, we
might want to consider both approaches.

Independent of that consideration, there is a great deal more work that
could be done to make more kinds of PARAM_EXEC parameters parallel-safe.
This infrastructure could be used to allow a Gather (Merge) on the inner
side of a nested loop (although that's not a very appealing plan) and
cases where the InitPlan is attached below the Gather (Merge) could be
addressed as well using various techniques.  But this is a good start.

Amit Kapila, reviewed and revised by me.  Reviewing and testing from
Kuntal Ghosh, Haribabu Kommi, and Tushar Ahuja.

Discussion: http://postgr.es/m/CAA4eK1LV0Y1AUV4cUCdC+sYOx0Z0-8NAJ2Pd9=UKsbQ5Sr7+JQ@mail.gmail.com

1 files changed, 50 insertions, 1 deletions

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index fa9a3f0b47b..28a7f7ec456 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -107,6 +107,7 @@ static Node *fix_scan_expr_mutator(Node *node, fix_scan_expr_context *context);
 static bool fix_scan_expr_walker(Node *node, fix_scan_expr_context *context);
 static void set_join_references(PlannerInfo *root, Join *join, int rtoffset);
 static void set_upper_references(PlannerInfo *root, Plan *plan, int rtoffset);
+static void set_param_references(PlannerInfo *root, Plan *plan);
 static Node *convert_combining_aggrefs(Node *node, void *context);
 static void set_dummy_tlist_references(Plan *plan, int rtoffset);
 static indexed_tlist *build_tlist_index(List *tlist);
@@ -632,7 +633,10 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 
 		case T_Gather:
 		case T_GatherMerge:
-			set_upper_references(root, plan, rtoffset);
+			{
+				set_upper_references(root, plan, rtoffset);
+				set_param_references(root, plan);
+			}
 			break;
 
 		case T_Hash:
@@ -1782,6 +1786,51 @@ set_upper_references(PlannerInfo *root, Plan *plan, int rtoffset)
 }
 
 /*
+ * set_param_references
+ *	  Initialize the initParam list in Gather or Gather merge node such that
+ *	  it contains reference of all the params that needs to be evaluated
+ *	  before execution of the node.  It contains the initplan params that are
+ *	  being passed to the plan nodes below it.
+ */
+static void
+set_param_references(PlannerInfo *root, Plan *plan)
+{
+	Assert(IsA(plan, Gather) || IsA(plan, GatherMerge));
+
+	if (plan->lefttree->extParam)
+	{
+		PlannerInfo *proot;
+		Bitmapset  *initSetParam = NULL;
+		ListCell   *l;
+
+		for (proot = root; proot != NULL; proot = proot->parent_root)
+		{
+			foreach(l, proot->init_plans)
+			{
+				SubPlan    *initsubplan = (SubPlan *) lfirst(l);
+				ListCell   *l2;
+
+				foreach(l2, initsubplan->setParam)
+				{
+					initSetParam = bms_add_member(initSetParam, lfirst_int(l2));
+				}
+			}
+		}
+
+		/*
+		 * Remember the list of all external initplan params that are used by
+		 * the children of Gather or Gather merge node.
+		 */
+		if (IsA(plan, Gather))
+			((Gather *) plan)->initParam =
+				bms_intersect(plan->lefttree->extParam, initSetParam);
+		else
+			((GatherMerge *) plan)->initParam =
+				bms_intersect(plan->lefttree->extParam, initSetParam);
+	}
+}
+
+/*
  * Recursively scan an expression tree and convert Aggrefs to the proper
  * intermediate form for combining aggregates.  This means (1) replacing each
  * one's argument list with a single argument that is the original Aggref