Disable parallel query by default.

Per discussion, set the default value of max_parallel_workers_per_gather
to 0 in 9.6 only.  We'll leave it enabled in master so that it gets
more testing and in the hope that it can be enable by default in v10.

Author: robertmhaas

doc/src/sgml/config.sgml

@@ -2008,7 +2008,7 @@ include_dir 'conf.d'
          <xref linkend="guc-max-worker-processes">.  Note that the requested
          number of workers may not actually be available at run time.  If this
          occurs, the plan will run with fewer workers than expected, which may
-         be inefficient.  The default value is 2.  Setting this value to 0
+         be inefficient.  Setting this value to 0, which is the default,
          disables parallel query execution.
         </para>
 

src/backend/optimizer/path/costsize.c

@@ -113,7 +113,7 @@ int			effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
 
 Cost		disable_cost = 1.0e10;
 
-int			max_parallel_workers_per_gather = 2;
+int			max_parallel_workers_per_gather = 0;
 
 bool		enable_seqscan = true;
 bool		enable_indexscan = true;

src/backend/utils/misc/guc.c

@@ -2652,7 +2652,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_parallel_workers_per_gather,
-		2, 0, 1024,
+		0, 0, 1024,
 		NULL, NULL, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -163,7 +163,7 @@
 
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching
 #max_worker_processes = 8		# (change requires restart)
-#max_parallel_workers_per_gather = 2	# taken from max_worker_processes
+#max_parallel_workers_per_gather = 0	# taken from max_worker_processes
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 									# (change requires restart)
 #backend_flush_after = 0		# 0 disables, default is 0