Extend ExecBuildAggTrans() to support a NULL pointer check.

Optionally push a step to check for a NULL pointer to the pergroup
state.

This will be important for disk-based hash aggregation in combination
with grouping sets. When memory limits are reached, a given tuple may
find its per-group state for some grouping sets but not others. For
the former, it advances the per-group state as normal; for the latter,
it skips evaluation and the calling code will have to spill the tuple
and reprocess it in a later batch.

Add the NULL check as a separate expression step because in some
common cases it's not needed.

Discussion: https://postgr.es/m/20200221202212.ssb2qpmdgrnx52sj%40alap3.anarazel.de

src/include/executor/execExpr.h

@@ -225,6 +225,7 @@ typedef enum ExprEvalOp
 	EEOP_AGG_DESERIALIZE,
 	EEOP_AGG_STRICT_INPUT_CHECK_ARGS,
 	EEOP_AGG_STRICT_INPUT_CHECK_NULLS,
+	EEOP_AGG_PLAIN_PERGROUP_NULLCHECK,
 	EEOP_AGG_PLAIN_TRANS_INIT_STRICT_BYVAL,
 	EEOP_AGG_PLAIN_TRANS_STRICT_BYVAL,
 	EEOP_AGG_PLAIN_TRANS_BYVAL,
@@ -622,6 +623,13 @@ typedef struct ExprEvalStep
 			int			jumpnull;
 		}			agg_strict_input_check;
 
+		/* for EEOP_AGG_PLAIN_PERGROUP_NULLCHECK */
+		struct
+		{
+			int			setoff;
+			int			jumpnull;
+		}			agg_plain_pergroup_nullcheck;
+
 		/* for EEOP_AGG_PLAIN_TRANS_[INIT_][STRICT_]{BYVAL,BYREF} */
 		/* for EEOP_AGG_ORDERED_TRANS_{DATUM,TUPLE} */
 		struct

src/include/executor/executor.h

@@ -255,7 +255,7 @@ extern ExprState *ExecInitQual(List *qual, PlanState *parent);
 extern ExprState *ExecInitCheck(List *qual, PlanState *parent);
 extern List *ExecInitExprList(List *nodes, PlanState *parent);
 extern ExprState *ExecBuildAggTrans(AggState *aggstate, struct AggStatePerPhaseData *phase,
-									bool doSort, bool doHash);
+									bool doSort, bool doHash, bool nullcheck);
 extern ExprState *ExecBuildGroupingEqual(TupleDesc ldesc, TupleDesc rdesc,
 										 const TupleTableSlotOps *lops, const TupleTableSlotOps *rops,
 										 int numCols,