Update Append's idea of first_partial_plan

It turns out that after runtime partition pruning, Append's
first_partial_plan does not accurately represent partial plans to run,
if any of those got pruned.  This could limit participation of workers
in some partial subplans, if other subplans got pruned.  Fix it by
keeping an index of the first valid partial subplan in the state node,
determined at execnode Init time.

Author: David Rowley, with cosmetic changes by me.
Discussion: https://postgr.es/m/CAKJS1f8o2Yd=rOP=Et3A0FWgF+gSAOkFSU6eNhnGzTPV7nN8sQ@mail.gmail.com

src/backend/executor/nodeAppend.c

@@ -104,6 +104,7 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 	PlanState **appendplanstates;
 	Bitmapset  *validsubplans;
 	int			nplans;
+	int			firstvalid;
 	int			i,
 				j;
 	ListCell   *lc;
@@ -207,19 +208,30 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 	/*
 	 * call ExecInitNode on each of the valid plans to be executed and save
 	 * the results into the appendplanstates array.
+	 *
+	 * While at it, find out the first valid partial plan.
 	 */
 	j = i = 0;
+	firstvalid = nplans;
 	foreach(lc, node->appendplans)
 	{
 		if (bms_is_member(i, validsubplans))
 		{
 			Plan	   *initNode = (Plan *) lfirst(lc);
 
+			/*
+			 * Record the lowest appendplans index which is a valid partial
+			 * plan.
+			 */
+			if (i >= node->first_partial_plan && j < firstvalid)
+				firstvalid = j;
+
 			appendplanstates[j++] = ExecInitNode(initNode, estate, eflags);
 		}
 		i++;
 	}
 
+	appendstate->as_first_partial_plan = firstvalid;
 	appendstate->appendplans = appendplanstates;
 	appendstate->as_nplans = nplans;
 
@@ -499,7 +511,6 @@ static bool
 choose_next_subplan_for_leader(AppendState *node)
 {
 	ParallelAppendState *pstate = node->as_pstate;
-	Append	   *append = (Append *) node->ps.plan;
 
 	/* Backward scan is not supported by parallel-aware plans */
 	Assert(ScanDirectionIsForward(node->ps.state->es_direction));
@@ -556,7 +567,7 @@ choose_next_subplan_for_leader(AppendState *node)
 	}
 
 	/* If non-partial, immediately mark as finished. */
-	if (node->as_whichplan < append->first_partial_plan)
+	if (node->as_whichplan < node->as_first_partial_plan)
 		node->as_pstate->pa_finished[node->as_whichplan] = true;
 
 	LWLockRelease(&pstate->pa_lock);
@@ -581,7 +592,6 @@ static bool
 choose_next_subplan_for_worker(AppendState *node)
 {
 	ParallelAppendState *pstate = node->as_pstate;
-	Append	   *append = (Append *) node->ps.plan;
 
 	/* Backward scan is not supported by parallel-aware plans */
 	Assert(ScanDirectionIsForward(node->ps.state->es_direction));
@@ -629,14 +639,14 @@ choose_next_subplan_for_worker(AppendState *node)
 			/* Advance to the next valid plan. */
 			pstate->pa_next_plan = nextplan;
 		}
-		else if (node->as_whichplan > append->first_partial_plan)
+		else if (node->as_whichplan > node->as_first_partial_plan)
 		{
 			/*
 			 * Try looping back to the first valid partial plan, if there is
 			 * one.  If there isn't, arrange to bail out below.
 			 */
 			nextplan = bms_next_member(node->as_valid_subplans,
-									   append->first_partial_plan - 1);
+									   node->as_first_partial_plan - 1);
 			pstate->pa_next_plan =
 				nextplan < 0 ? node->as_whichplan : nextplan;
 		}
@@ -670,7 +680,7 @@ choose_next_subplan_for_worker(AppendState *node)
 	if (pstate->pa_next_plan < 0)
 	{
 		int			nextplan = bms_next_member(node->as_valid_subplans,
-											   append->first_partial_plan - 1);
+											   node->as_first_partial_plan - 1);
 
 		if (nextplan >= 0)
 			pstate->pa_next_plan = nextplan;
@@ -686,7 +696,7 @@ choose_next_subplan_for_worker(AppendState *node)
 	}
 
 	/* If non-partial, immediately mark as finished. */
-	if (node->as_whichplan < append->first_partial_plan)
+	if (node->as_whichplan < node->as_first_partial_plan)
 		node->as_pstate->pa_finished[node->as_whichplan] = true;
 
 	LWLockRelease(&pstate->pa_lock);

src/include/nodes/execnodes.h

@@ -1087,6 +1087,8 @@ struct AppendState
 	PlanState **appendplans;	/* array of PlanStates for my inputs */
 	int			as_nplans;
 	int			as_whichplan;
+	int			as_first_partial_plan; /* Index of 'appendplans' containing
+										* the first partial plan */
 	ParallelAppendState *as_pstate; /* parallel coordination info */
 	Size		pstate_len;		/* size of parallel coordination info */
 	struct PartitionPruneState *as_prune_state;