diff --git a/src/backend/executor/nodeAgg.c b/src/backend/executor/nodeAgg.c
index f49114a..a2c29c4 100644
--- a/src/backend/executor/nodeAgg.c
+++ b/src/backend/executor/nodeAgg.c
@@ -756,11 +756,18 @@ advance_transition_function(AggState *aggstate,
 	aggstate->curpertrans = NULL;
 
 	/*
-	 * If pass-by-ref datatype, must copy the new value into aggcontext and
-	 * pfree the prior transValue.  But if transfn returned a pointer to its
-	 * first input, we don't need to do anything.
+	 * If we got a R/W pointer to an expanded object, we can just take over
+	 * control of the object.  Any other pass-by-ref value must be copied into
+	 * aggcontext and the prior value freed; with the exception that if transfn
+	 * returned a pointer to its first input, we don't need to do anything.
 	 */
-	if (!pertrans->transtypeByVal &&
+	if (DatumIsReadWriteExpandedObject(newVal, fcinfo->isnull,
+									   pertrans->transtypeLen))
+	{
+		newVal = TransferExpandedObject(newVal,
+		  aggstate->aggcontexts[aggstate->current_set]->ecxt_per_tuple_memory);
+	}
+	else if (!pertrans->transtypeByVal &&
 		DatumGetPointer(newVal) != DatumGetPointer(pergroupstate->transValue))
 	{
 		if (!fcinfo->isnull)
diff --git a/src/backend/utils/adt/Makefile b/src/backend/utils/adt/Makefile
index 2cb7bab..61c9359 100644
--- a/src/backend/utils/adt/Makefile
+++ b/src/backend/utils/adt/Makefile
@@ -12,7 +12,7 @@ include $(top_builddir)/src/Makefile.global
 OBJS = acl.o arrayfuncs.o array_expanded.o array_selfuncs.o \
 	array_typanalyze.o array_userfuncs.o arrayutils.o ascii.o \
 	bool.o cash.o char.o date.o datetime.o datum.o dbsize.o domains.o \
-	encode.o enum.o expandeddatum.o \
+	encode.o enum.o expandeddatum.o expandedstring.o \
 	float.o format_type.o formatting.o genfile.o \
 	geo_ops.o geo_selfuncs.o inet_cidr_ntop.o inet_net_pton.o int.o \
 	int8.o json.o jsonb.o jsonb_gin.o jsonb_op.o jsonb_util.o \
diff --git a/src/backend/utils/adt/expandedstring.c b/src/backend/utils/adt/expandedstring.c
new file mode 100644
index 0000000..4e279a3
--- /dev/null
+++ b/src/backend/utils/adt/expandedstring.c
@@ -0,0 +1,86 @@
+/*-------------------------------------------------------------------------
+ *
+ * expandedstring.c
+ *	  Expand a varlena into a StringInfo.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/adt/expandeddatum.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "utils/expandedstring.h"
+#include "utils/memutils.h"
+
+static Size ESI_get_flat_size(ExpandedObjectHeader *eohptr);
+static void ESI_flatten_into(ExpandedObjectHeader *eohptr,
+				 void *result, Size allocated_size);
+
+static const ExpandedObjectMethods ESI_methods =
+{
+	ESI_get_flat_size,
+	ESI_flatten_into
+};
+
+/*
+ * Construct an expanded datum consisting of an empty StringInfo.
+ *
+ * Caller must ensure that CurrentMemoryContext points to a context with
+ * a suitable lifetime.
+ */
+ExpandedStringInfoHeader *
+GetExpandedStringInfo(void)
+{
+	ExpandedStringInfoHeader *esih;
+	MemoryContext	objcxt;
+	MemoryContext	oldcxt;
+
+	objcxt = AllocSetContextCreate(CurrentMemoryContext,
+								   "stringinfo expanded object",
+								   ALLOCSET_SMALL_MINSIZE,
+								   ALLOCSET_SMALL_INITSIZE,
+								   ALLOCSET_DEFAULT_MAXSIZE);
+
+	oldcxt = MemoryContextSwitchTo(objcxt);	
+	esih = palloc(sizeof(ExpandedStringInfoHeader));
+	EOH_init_header(&esih->hdr, &ESI_methods, objcxt);
+	initStringInfo(&esih->buf);
+	MemoryContextSwitchTo(oldcxt);
+
+	return esih;
+}
+
+/*
+ * The space required to flatten a StringInfo back to a plain old varlena is
+ * just the number of bytes we have in the buffer, plus the size of a 4-byte
+ * header.  Even if the buffer is short, we can't flatten to a packed
+ * representation.
+ */
+static Size
+ESI_get_flat_size(ExpandedObjectHeader *eohptr)
+{
+	ExpandedStringInfoHeader *esih = (ExpandedStringInfoHeader *) eohptr;
+
+	return VARHDRSZ + esih->buf.len;
+}
+
+/*
+ * Flattening a StringInfo just involves copying the data into the allocated
+ * space.
+ */
+static void
+ESI_flatten_into(ExpandedObjectHeader *eohptr,
+				 void *result, Size allocated_size)
+{
+	ExpandedStringInfoHeader *esih = (ExpandedStringInfoHeader *) eohptr;
+
+	Assert(allocated_size == VARHDRSZ + esih->buf.len);
+	memcpy(VARDATA(result), esih->buf.data, esih->buf.len);
+	SET_VARSIZE(result, VARHDRSZ + esih->buf.len);
+}
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 8683188..09d7c7e 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -30,6 +30,7 @@
 #include "regex/regex.h"
 #include "utils/builtins.h"
 #include "utils/bytea.h"
+#include "utils/expandedstring.h"
 #include "utils/lsyscache.h"
 #include "utils/memutils.h"
 #include "utils/pg_locale.h"
@@ -4357,16 +4358,6 @@ pg_column_size(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32(result);
 }
 
-/*
- * string_agg - Concatenates values and returns string.
- *
- * Syntax: string_agg(value text, delimiter text) RETURNS text
- *
- * Note: Any NULL values are ignored. The first-call delimiter isn't
- * actually used at all, and on subsequent calls the delimiter precedes
- * the associated value.
- */
-
 /* subroutine to initialize state */
 static StringInfo
 makeStringAggState(FunctionCallInfo fcinfo)
@@ -4392,46 +4383,54 @@ makeStringAggState(FunctionCallInfo fcinfo)
 	return state;
 }
 
+/*
+ * string_agg - Concatenates values and returns string.
+ *
+ * Syntax: string_agg(value text, delimiter text) RETURNS text
+ *
+ * Note: Any NULL values are ignored. The first-call delimiter isn't
+ * actually used at all, and on subsequent calls the delimiter precedes
+ * the associated value.
+ */
 Datum
 string_agg_transfn(PG_FUNCTION_ARGS)
 {
-	StringInfo	state;
+	ExpandedStringInfoHeader *state;
 
-	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
-
-	/* Append the value unless null. */
-	if (!PG_ARGISNULL(1))
+	/* If the second argument is NULL, just return the first argument. */
+	if (PG_ARGISNULL(1))
 	{
-		/* On the first time through, we ignore the delimiter. */
-		if (state == NULL)
-			state = makeStringAggState(fcinfo);
-		else if (!PG_ARGISNULL(2))
-			appendStringInfoText(state, PG_GETARG_TEXT_PP(2));	/* delimiter */
-
-		appendStringInfoText(state, PG_GETARG_TEXT_PP(1));		/* value */
+		if (PG_ARGISNULL(0))
+			PG_RETURN_NULL();
+		PG_RETURN_DATUM(PG_GETARG_DATUM(0));
 	}
 
 	/*
-	 * The transition type for string_agg() is declared to be "internal",
-	 * which is a pass-by-value type the same size as a pointer.
+	 * Expand the first argument if needed; then, add any delimeter (unless
+	 * the first argument is NULL).
 	 */
-	PG_RETURN_POINTER(state);
-}
-
-Datum
-string_agg_finalfn(PG_FUNCTION_ARGS)
-{
-	StringInfo	state;
-
-	/* cannot be called directly because of internal-type argument */
-	Assert(AggCheckCallContext(fcinfo, NULL));
+	if (!PG_ARGISNULL(0) && VARATT_IS_EXTERNAL_EXPANDED_RW(PG_GETARG_DATUM(0)))
+	{
+        state = (ExpandedStringInfoHeader *) DatumGetEOHP(PG_GETARG_DATUM(0));
+		if (!PG_ARGISNULL(2))
+			appendStringInfoText(&state->buf, PG_GETARG_TEXT_PP(2));
+	}
+	else
+	{
+		state = GetExpandedStringInfo();
+		/* Note that if the transition state is NULL, we skip the delimiter. */
+		if (!PG_ARGISNULL(0))
+		{
+			appendStringInfoText(&state->buf, PG_GETARG_TEXT_PP(0));
+			if (!PG_ARGISNULL(2))
+				appendStringInfoText(&state->buf, PG_GETARG_TEXT_PP(2));
+		}
+	}
 
-	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
+	/* Append second argument to first. */
+	appendStringInfoText(&state->buf, PG_GETARG_TEXT_PP(1));
 
-	if (state != NULL)
-		PG_RETURN_TEXT_P(cstring_to_text_with_len(state->data, state->len));
-	else
-		PG_RETURN_NULL();
+	PG_RETURN_DATUM(EOHPGetRWDatum(&state->hdr));
 }
 
 /*
diff --git a/src/include/catalog/pg_aggregate.h b/src/include/catalog/pg_aggregate.h
index 28b0669..d8f9649 100644
--- a/src/include/catalog/pg_aggregate.h
+++ b/src/include/catalog/pg_aggregate.h
@@ -279,7 +279,7 @@ DATA(insert ( 2335	n 0 array_agg_transfn	array_agg_finalfn	-				-				-				t f 0
 DATA(insert ( 4053	n 0 array_agg_array_transfn array_agg_array_finalfn -		-				-				t f 0	2281	0	0		0	_null_ _null_ ));
 
 /* text */
-DATA(insert ( 3538	n 0 string_agg_transfn	string_agg_finalfn	-				-				-				f f 0	2281	0	0		0	_null_ _null_ ));
+DATA(insert ( 3538	n 0 string_agg_transfn	-		-				-				-				f f 0	25 0	0		0	_null_ _null_ ));
 
 /* bytea */
 DATA(insert ( 3545	n 0 bytea_string_agg_transfn	bytea_string_agg_finalfn	-				-				-		f f 0	2281	0	0		0	_null_ _null_ ));
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index f58672e..6def62d 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -2612,10 +2612,8 @@ DESCR("aggregate final function");
 DATA(insert OID = 2817 (  float8_corr				PGNSP PGUID 12 1 0 0 0 f f f f t f i s 1 0 701 "1022" _null_ _null_ _null_ _null_ _null_ float8_corr _null_ _null_ _null_ ));
 DESCR("aggregate final function");
 
-DATA(insert OID = 3535 (  string_agg_transfn		PGNSP PGUID 12 1 0 0 0 f f f f f f i s 3 0 2281 "2281 25 25" _null_ _null_ _null_ _null_ _null_ string_agg_transfn _null_ _null_ _null_ ));
+DATA(insert OID = 3535 (  string_agg_transfn		PGNSP PGUID 12 1 0 0 0 f f f f f f i s 3 0 25 "25 25 25" _null_ _null_ _null_ _null_ _null_ string_agg_transfn _null_ _null_ _null_ ));
 DESCR("aggregate transition function");
-DATA(insert OID = 3536 (  string_agg_finalfn		PGNSP PGUID 12 1 0 0 0 f f f f f f i s 1 0 25 "2281" _null_ _null_ _null_ _null_ _null_ string_agg_finalfn _null_ _null_ _null_ ));
-DESCR("aggregate final function");
 DATA(insert OID = 3538 (  string_agg				PGNSP PGUID 12 1 0 0 0 t f f f f f i s 2 0 25 "25 25" _null_ _null_ _null_ _null_ _null_ aggregate_dummy _null_ _null_ _null_ ));
 DESCR("concatenate aggregate input into a string");
 DATA(insert OID = 3543 (  bytea_string_agg_transfn	PGNSP PGUID 12 1 0 0 0 f f f f f f i s 3 0 2281 "2281 17 17" _null_ _null_ _null_ _null_ _null_ bytea_string_agg_transfn _null_ _null_ _null_ ));
diff --git a/src/include/utils/builtins.h b/src/include/utils/builtins.h
index b35d206..4af77b9 100644
--- a/src/include/utils/builtins.h
+++ b/src/include/utils/builtins.h
@@ -841,7 +841,6 @@ extern Datum pg_column_size(PG_FUNCTION_ARGS);
 extern Datum bytea_string_agg_transfn(PG_FUNCTION_ARGS);
 extern Datum bytea_string_agg_finalfn(PG_FUNCTION_ARGS);
 extern Datum string_agg_transfn(PG_FUNCTION_ARGS);
-extern Datum string_agg_finalfn(PG_FUNCTION_ARGS);
 
 extern Datum text_concat(PG_FUNCTION_ARGS);
 extern Datum text_concat_ws(PG_FUNCTION_ARGS);
diff --git a/src/include/utils/expandedstring.h b/src/include/utils/expandedstring.h
new file mode 100644
index 0000000..bb03d89
--- /dev/null
+++ b/src/include/utils/expandedstring.h
@@ -0,0 +1,31 @@
+/*-------------------------------------------------------------------------
+ *
+ * expandedstring.h
+ *	  Expand a varlena into a StringInfo for faster in-memory manipulation.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/utils/expandedstring.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef EXPANDEDSTRING_H
+#define EXPANDEDSTRING_H
+
+#include "lib/stringinfo.h"
+#include "utils/expandeddatum.h"
+
+typedef struct ExpandedStringInfoHeader
+{
+	/* Standard header for expanded objects */
+	ExpandedObjectHeader hdr;
+
+	/* String info */
+	StringInfoData		buf;
+} ExpandedStringInfoHeader;
+
+extern ExpandedStringInfoHeader *GetExpandedStringInfo(void);
+
+#endif