Save a few cycles in advance_transition_function().

Keep a pre-initialized FunctionCallInfoData in AggStatePerAggData, and
re-use that at each row instead of doing InitFunctionCallInfoData each
time.  This saves only half a dozen assignments and maybe some stack
manipulation, and yet that seems to be good for a percent or two of the
overall query run time for simple aggregates such as count(*).  The cost
is that the FunctionCallInfoData (which is about a kilobyte, on 64-bit
machines) stays allocated for the duration of the query instead of being
short-lived stack data.  But we're already paying an equivalent space cost
for each regular FuncExpr or OpExpr node, so I don't feel bad about paying
it for aggregate functions.  The code seems a little cleaner this way too,
since the number of things passed to advance_transition_function decreases.

src/backend/executor/nodeAgg.c

@@ -235,6 +235,14 @@ typedef struct AggStatePerAggData
 	 */
 
 	Tuplesortstate *sortstate;	/* sort object, if DISTINCT or ORDER BY */
+
+	/*
+	 * This field is a pre-initialized FunctionCallInfo struct used for
+	 * calling this aggregate's transfn.  We save a few cycles per row by not
+	 * re-initializing the unchanging fields; which isn't much, but it seems
+	 * worth the extra space consumption.
+	 */
+	FunctionCallInfoData transfn_fcinfo;
 }	AggStatePerAggData;
 
 /*
@@ -290,8 +298,7 @@ static void initialize_aggregates(AggState *aggstate,
 					  AggStatePerGroup pergroup);
 static void advance_transition_function(AggState *aggstate,
 							AggStatePerAgg peraggstate,
-							AggStatePerGroup pergroupstate,
-							FunctionCallInfoData *fcinfo);
+							AggStatePerGroup pergroupstate);
 static void advance_aggregates(AggState *aggstate, AggStatePerGroup pergroup);
 static void process_ordered_aggregate_single(AggState *aggstate,
 								 AggStatePerAgg peraggstate,
@@ -399,21 +406,20 @@ initialize_aggregates(AggState *aggstate,
  * Given new input value(s), advance the transition function of an aggregate.
  *
  * The new values (and null flags) have been preloaded into argument positions
- * 1 and up in fcinfo, so that we needn't copy them again to pass to the
- * transition function.  No other fields of fcinfo are assumed valid.
+ * 1 and up in peraggstate->transfn_fcinfo, so that we needn't copy them again
+ * to pass to the transition function.	We also expect that the static fields
+ * of the fcinfo are already initialized; that was done by ExecInitAgg().
  *
  * It doesn't matter which memory context this is called in.
  */
 static void
 advance_transition_function(AggState *aggstate,
 							AggStatePerAgg peraggstate,
-							AggStatePerGroup pergroupstate,
-							FunctionCallInfoData *fcinfo)
+							AggStatePerGroup pergroupstate)
 {
-	int			numTransInputs = peraggstate->numTransInputs;
+	FunctionCallInfo fcinfo = &peraggstate->transfn_fcinfo;
 	MemoryContext oldContext;
 	Datum		newVal;
-	int			i;
 
 	if (peraggstate->transfn.fn_strict)
 	{
@@ -421,6 +427,9 @@ advance_transition_function(AggState *aggstate,
 		 * For a strict transfn, nothing happens when there's a NULL input; we
 		 * just keep the prior transValue.
 		 */
+		int			numTransInputs = peraggstate->numTransInputs;
+		int			i;
+
 		for (i = 1; i <= numTransInputs; i++)
 		{
 			if (fcinfo->argnull[i])
@@ -467,12 +476,9 @@ advance_transition_function(AggState *aggstate,
 	/*
 	 * OK to call the transition function
 	 */
-	InitFunctionCallInfoData(*fcinfo, &(peraggstate->transfn),
-							 numTransInputs + 1,
-							 peraggstate->aggCollation,
-							 (void *) aggstate, NULL);
 	fcinfo->arg[0] = pergroupstate->transValue;
 	fcinfo->argnull[0] = pergroupstate->transValueIsNull;
+	fcinfo->isnull = false;		/* just in case transfn doesn't set it */
 
 	newVal = FunctionCallInvoke(fcinfo);
 
@@ -574,19 +580,18 @@ advance_aggregates(AggState *aggstate, AggStatePerGroup pergroup)
 		else
 		{
 			/* We can apply the transition function immediately */
-			FunctionCallInfoData fcinfo;
+			FunctionCallInfo fcinfo = &peraggstate->transfn_fcinfo;
 
 			/* Load values into fcinfo */
 			/* Start from 1, since the 0th arg will be the transition value */
 			Assert(slot->tts_nvalid >= numTransInputs);
 			for (i = 0; i < numTransInputs; i++)
 			{
-				fcinfo.arg[i + 1] = slot->tts_values[i];
-				fcinfo.argnull[i + 1] = slot->tts_isnull[i];
+				fcinfo->arg[i + 1] = slot->tts_values[i];
+				fcinfo->argnull[i + 1] = slot->tts_isnull[i];
 			}
 
-			advance_transition_function(aggstate, peraggstate, pergroupstate,
-										&fcinfo);
+			advance_transition_function(aggstate, peraggstate, pergroupstate);
 		}
 	}
 }
@@ -622,17 +627,17 @@ process_ordered_aggregate_single(AggState *aggstate,
 	MemoryContext workcontext = aggstate->tmpcontext->ecxt_per_tuple_memory;
 	MemoryContext oldContext;
 	bool		isDistinct = (peraggstate->numDistinctCols > 0);
+	FunctionCallInfo fcinfo = &peraggstate->transfn_fcinfo;
 	Datum	   *newVal;
 	bool	   *isNull;
-	FunctionCallInfoData fcinfo;
 
 	Assert(peraggstate->numDistinctCols < 2);
 
 	tuplesort_performsort(peraggstate->sortstate);
 
 	/* Load the column into argument 1 (arg 0 will be transition value) */
-	newVal = fcinfo.arg + 1;
-	isNull = fcinfo.argnull + 1;
+	newVal = fcinfo->arg + 1;
+	isNull = fcinfo->argnull + 1;
 
 	/*
 	 * Note: if input type is pass-by-ref, the datums returned by the sort are
@@ -668,8 +673,7 @@ process_ordered_aggregate_single(AggState *aggstate,
 		}
 		else
 		{
-			advance_transition_function(aggstate, peraggstate, pergroupstate,
-										&fcinfo);
+			advance_transition_function(aggstate, peraggstate, pergroupstate);
 			/* forget the old value, if any */
 			if (!oldIsNull && !peraggstate->inputtypeByVal)
 				pfree(DatumGetPointer(oldVal));
@@ -704,7 +708,7 @@ process_ordered_aggregate_multi(AggState *aggstate,
 								AggStatePerGroup pergroupstate)
 {
 	MemoryContext workcontext = aggstate->tmpcontext->ecxt_per_tuple_memory;
-	FunctionCallInfoData fcinfo;
+	FunctionCallInfo fcinfo = &peraggstate->transfn_fcinfo;
 	TupleTableSlot *slot1 = peraggstate->evalslot;
 	TupleTableSlot *slot2 = peraggstate->uniqslot;
 	int			numTransInputs = peraggstate->numTransInputs;
@@ -739,12 +743,11 @@ process_ordered_aggregate_multi(AggState *aggstate,
 			/* Start from 1, since the 0th arg will be the transition value */
 			for (i = 0; i < numTransInputs; i++)
 			{
-				fcinfo.arg[i + 1] = slot1->tts_values[i];
-				fcinfo.argnull[i + 1] = slot1->tts_isnull[i];
+				fcinfo->arg[i + 1] = slot1->tts_values[i];
+				fcinfo->argnull[i + 1] = slot1->tts_isnull[i];
 			}
 
-			advance_transition_function(aggstate, peraggstate, pergroupstate,
-										&fcinfo);
+			advance_transition_function(aggstate, peraggstate, pergroupstate);
 
 			if (numDistinctCols > 0)
 			{
@@ -1799,6 +1802,7 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 								&transfnexpr,
 								&finalfnexpr);
 
+		/* set up infrastructure for calling the transfn and finalfn */
 		fmgr_info(transfn_oid, &peraggstate->transfn);
 		fmgr_info_set_expr((Node *) transfnexpr, &peraggstate->transfn);
 
@@ -1810,6 +1814,13 @@ ExecInitAgg(Agg *node, EState *estate, int eflags)
 
 		peraggstate->aggCollation = aggref->inputcollid;
 
+		InitFunctionCallInfoData(peraggstate->transfn_fcinfo,
+								 &peraggstate->transfn,
+								 peraggstate->numTransInputs + 1,
+								 peraggstate->aggCollation,
+								 (void *) aggstate, NULL);
+
+		/* get info about relevant datatypes */
 		get_typlenbyval(aggref->aggtype,
 						&peraggstate->resulttypeLen,
 						&peraggstate->resulttypeByVal);