Allow parallel aggregate on string_agg and array_agg

This adds combine, serial and deserial functions for the array_agg() and
string_agg() aggregate functions, thus allowing these aggregates to
partake in partial aggregations.  This allows both parallel aggregation to
take place when these aggregates are present and also allows additional
partition-wise aggregation plan shapes to include plans that require
additional aggregation once the partially aggregated results from the
partitions have been combined.

Author: David Rowley
Reviewed-by: Andres Freund, Tomas Vondra, Stephen Frost, Tom Lane
Discussion: https://postgr.es/m/CAKJS1f9sx_6GTcvd6TMuZnNtCh0VhBzhX6FZqw17TgVFH-ga_A@mail.gmail.com

Author: david-rowley

doc/src/sgml/func.sgml

@@ -19746,7 +19746,7 @@ SELECT NULLIF(value, '(none)') ...
        <para>
         Collects all the input values, including nulls, into an array.
        </para></entry>
-       <entry>No</entry>
+       <entry>Yes</entry>
       </row>
 
       <row>
@@ -19759,7 +19759,7 @@ SELECT NULLIF(value, '(none)') ...
         dimension.  (The inputs must all have the same dimensionality, and
         cannot be empty or null.)
        </para></entry>
-       <entry>No</entry>
+       <entry>Yes</entry>
       </row>
 
       <row>
@@ -20099,7 +20099,7 @@ SELECT NULLIF(value, '(none)') ...
         after the first is preceded by the
         corresponding <parameter>delimiter</parameter> (if it's not null).
        </para></entry>
-       <entry>No</entry>
+       <entry>Yes</entry>
       </row>
 
       <row>

src/backend/optimizer/prep/prepagg.c

@@ -305,10 +305,30 @@ preprocess_aggref(Aggref *aggref, PlannerInfo *root)
 				 * functions; if not, we can't serialize partial-aggregation
 				 * results.
 				 */
-				else if (transinfo->aggtranstype == INTERNALOID &&
-						 (!OidIsValid(transinfo->serialfn_oid) ||
-						  !OidIsValid(transinfo->deserialfn_oid)))
-					root->hasNonSerialAggs = true;
+				else if (transinfo->aggtranstype == INTERNALOID)
+				{
+
+					if (!OidIsValid(transinfo->serialfn_oid) ||
+						!OidIsValid(transinfo->deserialfn_oid))
+						root->hasNonSerialAggs = true;
+
+					/*
+					 * array_agg_serialize and array_agg_deserialize make use
+					 * of the aggregate non-byval input type's send and
+					 * receive functions.  There's a chance that the type
+					 * being aggregated has one or both of these functions
+					 * missing.  In this case we must not allow the
+					 * aggregate's serial and deserial functions to be used.
+					 * It would be nice not to have special case this and
+					 * instead provide some sort of supporting function within
+					 * the aggregate to do this, but for now, that seems like
+					 * overkill for this one case.
+					 */
+					if ((transinfo->serialfn_oid == F_ARRAY_AGG_SERIALIZE ||
+						 transinfo->deserialfn_oid == F_ARRAY_AGG_DESERIALIZE) &&
+						!agg_args_support_sendreceive(aggref))
+						root->hasNonSerialAggs = true;
+				}
 			}
 		}
 		agginfo->transno = transno;

src/backend/parser/parse_agg.c

@@ -14,6 +14,7 @@
  */
 #include "postgres.h"
 
+#include "access/htup_details.h"
 #include "catalog/pg_aggregate.h"
 #include "catalog/pg_constraint.h"
 #include "catalog/pg_type.h"
@@ -28,7 +29,7 @@
 #include "rewrite/rewriteManip.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
-
+#include "utils/syscache.h"
 
 typedef struct
 {
@@ -1947,6 +1948,40 @@ resolve_aggregate_transtype(Oid aggfuncid,
 	return aggtranstype;
 }
 
+/*
+ * agg_args_support_sendreceive
+ *		Returns true if all non-byval of aggref's arg types have send and
+ *		receive functions.
+ */
+bool
+agg_args_support_sendreceive(Aggref *aggref)
+{
+	ListCell   *lc;
+
+	foreach(lc, aggref->args)
+	{
+		HeapTuple	typeTuple;
+		Form_pg_type pt;
+		TargetEntry *tle = (TargetEntry *) lfirst(lc);
+		Oid			type = exprType((Node *) tle->expr);
+
+		typeTuple = SearchSysCache1(TYPEOID, ObjectIdGetDatum(type));
+		if (!HeapTupleIsValid(typeTuple))
+			elog(ERROR, "cache lookup failed for type %u", type);
+
+		pt = (Form_pg_type) GETSTRUCT(typeTuple);
+
+		if (!pt->typbyval &&
+			(!OidIsValid(pt->typsend) || !OidIsValid(pt->typreceive)))
+		{
+			ReleaseSysCache(typeTuple);
+			return false;
+		}
+		ReleaseSysCache(typeTuple);
+	}
+	return true;
+}
+
 /*
  * Create an expression tree for the transition function of an aggregate.
  * This is needed so that polymorphic functions can be used within an