Don't call ExecAssignScanProjectionInfo while in partitionMemoryContext.

This allows removing the weird pfree() of the resultTupleSlot's tuple
descriptor. What would've happened without the pfree() is that the old
slot was allocated in the first ExecAssignScanProjectionInfo() call, in
partitionMemoryContext, and then immediately destroyed when the memory
context was reset. The second call to ExecAssignScanProjectionInfo() tries
to free the slot, again, causing the segfault. But we can avoid that
by this rearrangement of the calls in a cleaner way.

In the passing, clean up the code a bit. I found having separate variables,
indexState and scanState, which point to the same struct, to be confusing.

src/backend/executor/nodeDynamicIndexscan.c

@@ -142,8 +142,8 @@ static bool
 initNextIndexToScan(DynamicIndexScanState *node)
 {
 	IndexScanState *indexState = &(node->indexScanState);
-
 	DynamicIndexScan *dynamicIndexScan = (DynamicIndexScan *)node->indexScanState.ss.ps.plan;
+	EState *estate = indexState->ss.ps.state;
 
 	/* Load new index when the scanning of the previous index is done. */
 	if (indexState->ss.scan_state == SCAN_INIT ||
@@ -186,18 +186,18 @@ initNextIndexToScan(DynamicIndexScanState *node)
 
 		ExecAssignScanType(&indexState->ss, RelationGetDescr(currentRelation));
 
-		ScanState *scanState = (ScanState *)node;
+		/*
+		 * Initialize result tuple type and projection info.
+		 */
+		ExecAssignResultTypeFromTL(&indexState->ss.ps);
+		ExecAssignScanProjectionInfo(&indexState->ss);
 
 		MemoryContextReset(node->partitionMemoryContext);
 		MemoryContext oldCxt = MemoryContextSwitchTo(node->partitionMemoryContext);
 
 		/* Initialize child expressions */
-		scanState->ps.qual = (List *)ExecInitExpr((Expr *)scanState->ps.plan->qual, (PlanState*)scanState);
-		scanState->ps.targetlist = (List *)ExecInitExpr((Expr *)scanState->ps.plan->targetlist, (PlanState*)scanState);
-
-		ExecAssignScanProjectionInfo(scanState);
-
-		EState *estate = indexState->ss.ps.state;
+		indexState->ss.ps.qual = (List *) ExecInitExpr((Expr *) indexState->ss.ps.plan->qual, (PlanState *) indexState);
+		indexState->ss.ps.targetlist = (List *) ExecInitExpr((Expr *) indexState->ss.ps.plan->targetlist, (PlanState *) indexState);
 
 		indexState->iss_RelationDesc =
 			OpenIndexRelation(estate, pindex, *pid);
@@ -219,29 +219,14 @@ initNextIndexToScan(DynamicIndexScanState *node)
 
 		MemoryContextSwitchTo(oldCxt);
 
-		ExprContext *econtext = indexState->iss_RuntimeContext;		/* context for runtime keys */
-
 		if (indexState->iss_NumRuntimeKeys != 0)
 		{
-			ExecIndexEvalRuntimeKeys(econtext,
+			ExecIndexEvalRuntimeKeys(indexState->iss_RuntimeContext,
 									 indexState->iss_RuntimeKeys,
 									 indexState->iss_NumRuntimeKeys);
 		}
-
 		indexState->iss_RuntimeKeysReady = true;
 
-		/*
-		 * Initialize result tuple type and projection info.
-		 */
-		TupleDesc td = indexState->ss.ps.ps_ResultTupleSlot->tts_tupleDescriptor;
-		if (td)
-		{
-			pfree(td);
-			td = NULL;
-		}
-		ExecAssignResultTypeFromTL(&indexState->ss.ps);
-		ExecAssignScanProjectionInfo(&indexState->ss);
-
 		indexState->iss_ScanDesc = index_beginscan(currentRelation,
 				indexState->iss_RelationDesc,
 				estate->es_snapshot,