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

1 files changed, 19 insertions, 1 deletions

diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c
index 4943be8999e..c100152162e 100644
--- a/src/backend/utils/adt/arrayfuncs.c
+++ b/src/backend/utils/adt/arrayfuncs.c
@@ -5263,6 +5263,24 @@ array_insert_slice(ArrayType *destArray,
 ArrayBuildState *
 initArrayResult(Oid element_type, MemoryContext rcontext, bool subcontext)
 {
+	/*
+	 * When using a subcontext, we can afford to start with a somewhat larger
+	 * initial array size.  Without subcontexts, we'd better hope that most of
+	 * the states stay small ...
+	 */
+	return initArrayResultWithSize(element_type, rcontext, subcontext,
+								   subcontext ? 64 : 8);
+}
+
+/*
+ * initArrayResultWithSize
+ *		As initArrayResult, but allow the initial size of the allocated arrays
+ *		to be specified.
+ */
+ArrayBuildState *
+initArrayResultWithSize(Oid element_type, MemoryContext rcontext,
+						bool subcontext, int initsize)
+{
 	ArrayBuildState *astate;
 	MemoryContext arr_context = rcontext;
 
@@ -5276,7 +5294,7 @@ initArrayResult(Oid element_type, MemoryContext rcontext, bool subcontext)
 		MemoryContextAlloc(arr_context, sizeof(ArrayBuildState));
 	astate->mcontext = arr_context;
 	astate->private_cxt = subcontext;
-	astate->alen = (subcontext ? 64 : 8);	/* arbitrary starting array size */
+	astate->alen = initsize;
 	astate->dvalues = (Datum *)
 		MemoryContextAlloc(arr_context, astate->alen * sizeof(Datum));
 	astate->dnulls = (bool *)