Expression evaluation based aggregate transition invocation.

Previously aggregate transition and combination functions were invoked
by special case code in nodeAgg.c, evaluating input and filters
separately using the expression evaluation machinery. That turns out
to not be great for performance for several reasons:

- repeated expression evaluations have some cost
- the transition functions invocations are poorly predicted, as
  commonly there are multiple aggregates in a query, resulting in the
  same call-stack invoking different functions.
- filter and input computation had to be done separately
- the special case code made it hard to implement JITing of the whole
  transition function invocation

Address this by building one large expression that computes input,
evaluates filters, and invokes transition functions.

This leads to moderate speedups in queries bottlenecked by aggregate
computations, and enables large speedups for similar cases once JITing
is done.

There's potential for further improvement:
- It'd be nice if we could simplify the somewhat expensive
  aggstate->all_pergroups lookups.
- right now there's still an advance_transition_function invocation in
  nodeAgg.c, leading to some code duplication.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

src/include/executor/execExpr.h

@@ -14,6 +14,7 @@
 #ifndef EXEC_EXPR_H
 #define EXEC_EXPR_H
 
+#include "executor/nodeAgg.h"
 #include "nodes/execnodes.h"
 
 /* forward references to avoid circularity */
@@ -64,9 +65,9 @@ typedef enum ExprEvalOp
 	EEOP_WHOLEROW,
 
 	/*
-	 * Compute non-system Var value, assign it into ExprState's
-	 * resultslot. These are not used if a CheckVarSlotCompatibility() check
-	 * would be needed.
+	 * Compute non-system Var value, assign it into ExprState's resultslot.
+	 * These are not used if a CheckVarSlotCompatibility() check would be
+	 * needed.
 	 */
 	EEOP_ASSIGN_INNER_VAR,
 	EEOP_ASSIGN_OUTER_VAR,
@@ -218,6 +219,17 @@ typedef enum ExprEvalOp
 	EEOP_SUBPLAN,
 	EEOP_ALTERNATIVE_SUBPLAN,
 
+	/* aggregation related nodes */
+	EEOP_AGG_STRICT_DESERIALIZE,
+	EEOP_AGG_DESERIALIZE,
+	EEOP_AGG_STRICT_INPUT_CHECK,
+	EEOP_AGG_INIT_TRANS,
+	EEOP_AGG_STRICT_TRANS_CHECK,
+	EEOP_AGG_PLAIN_TRANS_BYVAL,
+	EEOP_AGG_PLAIN_TRANS,
+	EEOP_AGG_ORDERED_TRANS_DATUM,
+	EEOP_AGG_ORDERED_TRANS_TUPLE,
+
 	/* non-existent operation, used e.g. to check array lengths */
 	EEOP_LAST
 } ExprEvalOp;
@@ -573,6 +585,55 @@ typedef struct ExprEvalStep
 			/* out-of-line state, created by nodeSubplan.c */
 			AlternativeSubPlanState *asstate;
 		}			alternative_subplan;
+
+		/* for EEOP_AGG_*DESERIALIZE */
+		struct
+		{
+			AggState   *aggstate;
+			FunctionCallInfo fcinfo_data;
+			int			jumpnull;
+		}			agg_deserialize;
+
+		/* for EEOP_AGG_STRICT_INPUT_CHECK */
+		struct
+		{
+			bool	   *nulls;
+			int			nargs;
+			int			jumpnull;
+		}			agg_strict_input_check;
+
+		/* for EEOP_AGG_INIT_TRANS */
+		struct
+		{
+			AggState   *aggstate;
+			AggStatePerTrans pertrans;
+			ExprContext *aggcontext;
+			int			setno;
+			int			transno;
+			int			setoff;
+			int			jumpnull;
+		}			agg_init_trans;
+
+		/* for EEOP_AGG_STRICT_TRANS_CHECK */
+		struct
+		{
+			AggState   *aggstate;
+			int			setno;
+			int			transno;
+			int			setoff;
+			int			jumpnull;
+		}			agg_strict_trans_check;
+
+		/* for EEOP_AGG_{PLAIN,ORDERED}_TRANS* */
+		struct
+		{
+			AggState   *aggstate;
+			AggStatePerTrans pertrans;
+			ExprContext *aggcontext;
+			int			setno;
+			int			transno;
+			int			setoff;
+		}			agg_trans;
 	}			d;
 } ExprEvalStep;
 
@@ -669,4 +730,13 @@ extern void ExecEvalAlternativeSubPlan(ExprState *state, ExprEvalStep *op,
 extern void ExecEvalWholeRowVar(ExprState *state, ExprEvalStep *op,
 					ExprContext *econtext);
 
+extern void ExecAggInitGroup(AggState *aggstate, AggStatePerTrans pertrans, AggStatePerGroup pergroup);
+extern Datum ExecAggTransReparent(AggState *aggstate, AggStatePerTrans pertrans,
+					 Datum newValue, bool newValueIsNull,
+					 Datum oldValue, bool oldValueIsNull);
+extern void ExecEvalAggOrderedTransDatum(ExprState *state, ExprEvalStep *op,
+							 ExprContext *econtext);
+extern void ExecEvalAggOrderedTransTuple(ExprState *state, ExprEvalStep *op,
+							 ExprContext *econtext);
+
 #endif							/* EXEC_EXPR_H */