Add a GUC to limit the number of slices for a query

Executing a query plan containing a large number of slices may slow down
the entire Greenplum cluster: each "n-gang" slice corresponds to a
separate process per segment. An example of such queries is a UNION ALL
atop several complex views. To prevent such a situation, add a GUC
gp_max_slices and refuse to execute plans of which the number of slices
exceed that limit.
Signed-off-by: NJesse Zhang <sbjesse@gmail.com>

src/backend/executor/execUtils.c

@@ -1365,6 +1365,14 @@ InitSliceTable(EState *estate, int nMotions, int nSubplans)
 				n;
 	MemoryContext oldcontext;
 
+	n = 1 + nMotions + nSubplans;
+
+	if (gp_max_slices > 0 && n > gp_max_slices)
+		ereport(ERROR,
+				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
+				 errmsg("at most %d slices are allowed in a query, current number: %d", gp_max_slices, n),
+				 errhint("rewrite your query or adjust GUC gp_max_slices")));
+
 	oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
 
 	table = makeNode(SliceTable);
@@ -1376,7 +1384,6 @@ InitSliceTable(EState *estate, int nMotions, int nSubplans)
 	/* Each slice table has a unique-id. */
 	table->ic_instance_id = ++gp_interconnect_id;
 
-	n = 1 + nMotions + nSubplans;
 	for (i = 0; i < n; i++)
 	{
 		slice = makeNode(Slice);

src/backend/utils/misc/guc_gp.c

@@ -579,6 +579,9 @@ bool		optimizer_enable_associativity;
 bool		optimizer_analyze_root_partition;
 bool		optimizer_analyze_midlevel_partition;
 
+/* GUCs for slice table*/
+int			gp_max_slices;
+
 /* System Information */
 static int	gp_server_version_num;
 static char *gp_server_version_string;
@@ -4720,6 +4723,16 @@ struct config_int ConfigureNamesInt_gp[] =
 		GP_VERSION_NUM, GP_VERSION_NUM, GP_VERSION_NUM, NULL, NULL
 	},
 
+	{
+		{"gp_max_slices", PGC_USERSET, PRESET_OPTIONS,
+			gettext_noop("Maximum slices for a single query"),
+			NULL,
+			GUC_GPDB_ADDOPT | GUC_NOT_IN_SAMPLE
+		},
+		&gp_max_slices,
+		0, 0, INT_MAX, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL

src/include/utils/guc.h

@@ -500,6 +500,9 @@ extern bool optimizer_analyze_midlevel_partition;
 extern bool optimizer_use_gpdb_allocators;
 
 
+/* GUCs for slice table*/
+extern int	gp_max_slices;
+
 /**
  * Enable logging of DPE match in optimizer.
  */