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

src/backend/utils/adt/varlena.c

@@ -506,29 +506,50 @@ bytea_string_agg_transfn(PG_FUNCTION_ARGS)
 
 	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
 
-	/* Append the value unless null. */
+	/* Append the value unless null, preceding it with the delimiter. */
 	if (!PG_ARGISNULL(1))
 	{
 		bytea	   *value = PG_GETARG_BYTEA_PP(1);
+		bool		isfirst = false;
 
-		/* On the first time through, we ignore the delimiter. */
+		/*
+		 * You might think we can just throw away the first delimiter, however
+		 * we must keep it as we may be a parallel worker doing partial
+		 * aggregation building a state to send to the main process.  We need
+		 * to keep the delimiter of every aggregation so that the combine
+		 * function can properly join up the strings of two separately
+		 * partially aggregated results.  The first delimiter is only stripped
+		 * off in the final function.  To know how much to strip off the front
+		 * of the string, we store the length of the first delimiter in the
+		 * StringInfo's cursor field, which we don't otherwise need here.
+		 */
 		if (state == NULL)
+		{
 			state = makeStringAggState(fcinfo);
-		else if (!PG_ARGISNULL(2))
+			isfirst = true;
+		}
+
+		if (!PG_ARGISNULL(2))
 		{
 			bytea	   *delim = PG_GETARG_BYTEA_PP(2);
 
-			appendBinaryStringInfo(state, VARDATA_ANY(delim), VARSIZE_ANY_EXHDR(delim));
+			appendBinaryStringInfo(state, VARDATA_ANY(delim),
+								   VARSIZE_ANY_EXHDR(delim));
+			if (isfirst)
+				state->cursor = VARSIZE_ANY_EXHDR(delim);
 		}
 
-		appendBinaryStringInfo(state, VARDATA_ANY(value), VARSIZE_ANY_EXHDR(value));
+		appendBinaryStringInfo(state, VARDATA_ANY(value),
+							   VARSIZE_ANY_EXHDR(value));
 	}
 
 	/*
 	 * The transition type for string_agg() is declared to be "internal",
 	 * which is a pass-by-value type the same size as a pointer.
 	 */
-	PG_RETURN_POINTER(state);
+	if (state)
+		PG_RETURN_POINTER(state);
+	PG_RETURN_NULL();
 }
 
 Datum
@@ -543,11 +564,13 @@ bytea_string_agg_finalfn(PG_FUNCTION_ARGS)
 
 	if (state != NULL)
 	{
+		/* As per comment in transfn, strip data before the cursor position */
 		bytea	   *result;
+		int			strippedlen = state->len - state->cursor;
 
-		result = (bytea *) palloc(state->len + VARHDRSZ);
-		SET_VARSIZE(result, state->len + VARHDRSZ);
-		memcpy(VARDATA(result), state->data, state->len);
+		result = (bytea *) palloc(strippedlen + VARHDRSZ);
+		SET_VARSIZE(result, strippedlen + VARHDRSZ);
+		memcpy(VARDATA(result), &state->data[state->cursor], strippedlen);
 		PG_RETURN_BYTEA_P(result);
 	}
 	else
@@ -5372,23 +5395,171 @@ string_agg_transfn(PG_FUNCTION_ARGS)
 
 	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
 
-	/* Append the value unless null. */
+	/* Append the value unless null, preceding it with the delimiter. */
 	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 */
+		text	   *value = PG_GETARG_TEXT_PP(1);
+		bool		isfirst = false;
 
-		appendStringInfoText(state, PG_GETARG_TEXT_PP(1));	/* value */
+		/*
+		 * You might think we can just throw away the first delimiter, however
+		 * we must keep it as we may be a parallel worker doing partial
+		 * aggregation building a state to send to the main process.  We need
+		 * to keep the delimiter of every aggregation so that the combine
+		 * function can properly join up the strings of two separately
+		 * partially aggregated results.  The first delimiter is only stripped
+		 * off in the final function.  To know how much to strip off the front
+		 * of the string, we store the length of the first delimiter in the
+		 * StringInfo's cursor field, which we don't otherwise need here.
+		 */
+		if (state == NULL)
+		{
+			state = makeStringAggState(fcinfo);
+			isfirst = true;
+		}
+
+		if (!PG_ARGISNULL(2))
+		{
+			text	   *delim = PG_GETARG_TEXT_PP(2);
+
+			appendStringInfoText(state, delim);
+			if (isfirst)
+				state->cursor = VARSIZE_ANY_EXHDR(delim);
+		}
+
+		appendStringInfoText(state, value);
 	}
 
 	/*
 	 * The transition type for string_agg() is declared to be "internal",
 	 * which is a pass-by-value type the same size as a pointer.
 	 */
-	PG_RETURN_POINTER(state);
+	if (state)
+		PG_RETURN_POINTER(state);
+	PG_RETURN_NULL();
+}
+
+/*
+ * string_agg_combine
+ *		Aggregate combine function for string_agg(text) and string_agg(bytea)
+ */
+Datum
+string_agg_combine(PG_FUNCTION_ARGS)
+{
+	StringInfo	state1;
+	StringInfo	state2;
+	MemoryContext agg_context;
+
+	if (!AggCheckCallContext(fcinfo, &agg_context))
+		elog(ERROR, "aggregate function called in non-aggregate context");
+
+	state1 = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
+	state2 = PG_ARGISNULL(1) ? NULL : (StringInfo) PG_GETARG_POINTER(1);
+
+	if (state2 == NULL)
+	{
+		/*
+		 * NULL state2 is easy, just return state1, which we know is already
+		 * in the agg_context
+		 */
+		if (state1 == NULL)
+			PG_RETURN_NULL();
+		PG_RETURN_POINTER(state1);
+	}
+
+	if (state1 == NULL)
+	{
+		/* We must copy state2's data into the agg_context */
+		MemoryContext old_context;
+
+		old_context = MemoryContextSwitchTo(agg_context);
+		state1 = makeStringAggState(fcinfo);
+		appendBinaryStringInfo(state1, state2->data, state2->len);
+		state1->cursor = state2->cursor;
+		MemoryContextSwitchTo(old_context);
+	}
+	else if (state2->len > 0)
+	{
+		/* Combine ... state1->cursor does not change in this case */
+		appendBinaryStringInfo(state1, state2->data, state2->len);
+	}
+
+	PG_RETURN_POINTER(state1);
+}
+
+/*
+ * string_agg_serialize
+ *		Aggregate serialize function for string_agg(text) and string_agg(bytea)
+ *
+ * This is strict, so we need not handle NULL input
+ */
+Datum
+string_agg_serialize(PG_FUNCTION_ARGS)
+{
+	StringInfo	state;
+	StringInfoData buf;
+	bytea	   *result;
+
+	/* cannot be called directly because of internal-type argument */
+	Assert(AggCheckCallContext(fcinfo, NULL));
+
+	state = (StringInfo) PG_GETARG_POINTER(0);
+
+	pq_begintypsend(&buf);
+
+	/* cursor */
+	pq_sendint(&buf, state->cursor, 4);
+
+	/* data */
+	pq_sendbytes(&buf, state->data, state->len);
+
+	result = pq_endtypsend(&buf);
+
+	PG_RETURN_BYTEA_P(result);
+}
+
+/*
+ * string_agg_deserialize
+ *		Aggregate deserial function for string_agg(text) and string_agg(bytea)
+ *
+ * This is strict, so we need not handle NULL input
+ */
+Datum
+string_agg_deserialize(PG_FUNCTION_ARGS)
+{
+	bytea	   *sstate;
+	StringInfo	result;
+	StringInfoData buf;
+	char	   *data;
+	int			datalen;
+
+	/* cannot be called directly because of internal-type argument */
+	Assert(AggCheckCallContext(fcinfo, NULL));
+
+	sstate = PG_GETARG_BYTEA_PP(0);
+
+	/*
+	 * Copy the bytea into a StringInfo so that we can "receive" it using the
+	 * standard recv-function infrastructure.
+	 */
+	initStringInfo(&buf);
+	appendBinaryStringInfo(&buf,
+						   VARDATA_ANY(sstate), VARSIZE_ANY_EXHDR(sstate));
+
+	result = makeStringAggState(fcinfo);
+
+	/* cursor */
+	result->cursor = pq_getmsgint(&buf, 4);
+
+	/* data */
+	datalen = VARSIZE_ANY_EXHDR(sstate) - 4;
+	data = (char *) pq_getmsgbytes(&buf, datalen);
+	appendBinaryStringInfo(result, data, datalen);
+
+	pq_getmsgend(&buf);
+	pfree(buf.data);
+
+	PG_RETURN_POINTER(result);
 }
 
 Datum
@@ -5402,7 +5573,11 @@ string_agg_finalfn(PG_FUNCTION_ARGS)
 	state = PG_ARGISNULL(0) ? NULL : (StringInfo) PG_GETARG_POINTER(0);
 
 	if (state != NULL)
-		PG_RETURN_TEXT_P(cstring_to_text_with_len(state->data, state->len));
+	{
+		/* As per comment in transfn, strip data before the cursor position */
+		PG_RETURN_TEXT_P(cstring_to_text_with_len(&state->data[state->cursor],
+												  state->len - state->cursor));
+	}
 	else
 		PG_RETURN_NULL();
 }