Rethink node-level representation of partial-aggregation modes.

The original coding had three separate booleans representing partial
aggregation behavior, which was confusing, unreadable, and error-prone,
not least because the booleans weren't always listed in the same order.
It was also inadequate for the allegedly-desirable future extension to
support intermediate partial aggregation, because we'd need separate
markers for serialization and deserialization in such a case.

Merge these bools into an enum "AggSplit" to provide symbolic names for
the supported operating modes (and document what those are).  By assigning
the values of the enum constants carefully, we can treat AggSplit values
as options bitmasks so that tests of what to do aren't noticeably more
expensive than before.

While at it, get rid of Aggref.aggoutputtype.  That's not needed since
commit 59a3795c2 got rid of setrefs.c's special-purpose Aggref comparison
code, and it likewise seemed more confusing than helpful.

Assorted comment cleanup as well (there's still more that I want to do
in that line).

catversion bump for change in Aggref node contents.  Should be the last
one for partial-aggregation changes.

Discussion: <29309.1466699160@sss.pgh.pa.us>

src/include/nodes/primnodes.h

@@ -266,22 +266,18 @@ typedef struct Param
  * replaced with a single argument representing the partial-aggregate
  * transition values.
  *
- * XXX need more documentation about partial aggregation here
- *
- * 'aggtype' and 'aggoutputtype' are the same except when we're performing
- * partal aggregation; in that case, we output transition states.  Nothing
- * interesting happens in the Aggref itself, but we must set the output data
- * type to whatever type is used for transition values.
- *
- * Note: If you are adding fields here you may also need to add a comparison
- * in search_indexed_tlist_for_partial_aggref()
+ * aggsplit indicates the expected partial-aggregation mode for the Aggref's
+ * parent plan node.  It's always set to AGGSPLIT_SIMPLE in the parser, but
+ * the planner might change it to something else.  We use this mainly as
+ * a crosscheck that the Aggrefs match the plan; but note that when aggsplit
+ * indicates a non-final mode, aggtype reflects the transition data type
+ * not the SQL-level output type of the aggregate.
  */
 typedef struct Aggref
 {
 	Expr		xpr;
 	Oid			aggfnoid;		/* pg_proc Oid of the aggregate */
-	Oid			aggtype;		/* type Oid of final result of the aggregate */
-	Oid			aggoutputtype;	/* type Oid of result of this aggregate */
+	Oid			aggtype;		/* type Oid of result of the aggregate */
 	Oid			aggcollid;		/* OID of collation of result */
 	Oid			inputcollid;	/* OID of collation that function should use */
 	Oid			aggtranstype;	/* type Oid of aggregate's transition value */
@@ -294,10 +290,9 @@ typedef struct Aggref
 	bool		aggstar;		/* TRUE if argument list was really '*' */
 	bool		aggvariadic;	/* true if variadic arguments have been
 								 * combined into an array last argument */
-	bool		aggcombine;		/* combining agg; input is a transvalue */
-	bool		aggpartial;		/* partial agg; output is a transvalue */
 	char		aggkind;		/* aggregate kind (see pg_aggregate.h) */
 	Index		agglevelsup;	/* > 0 if agg belongs to outer query */
+	AggSplit	aggsplit;		/* expected agg-splitting mode of parent Agg */
 	int			location;		/* token location, or -1 if unknown */
 } Aggref;