Perform one only projection to compute agg arguments.

Previously we did a ExecProject() for each individual aggregate
argument. That turned out to be a performance bottleneck in queries with
multiple aggregates.

Doing all the argument computations in one ExecProject() is quite a bit
cheaper because ExecProject's fastpath can do the work at once in a
relatively tight loop, and because it can get all the required columns
with a single slot_getsomeattr and save some other redundant setup
costs.

Author: Andres Freund
Reviewed-By: Heikki Linnakangas
Discussion: https://postgr.es/m/20161103110721.h5i5t5saxfk5eeik@alap3.anarazel.de

src/include/nodes/execnodes.h

@@ -1863,6 +1863,10 @@ typedef struct AggState
 	List	   *hash_needed;	/* list of columns needed in hash table */
 	bool		table_filled;	/* hash table filled yet? */
 	TupleHashIterator hashiter; /* for iterating through hash table */
+	/* support for evaluation of agg inputs */
+	TupleTableSlot *evalslot;	/* slot for agg inputs */
+	ProjectionInfo *evalproj;	/* projection machinery */
+	TupleDesc	evaldesc;		/* descriptor of input tuples */
 } AggState;
 
 /* ----------------