Enable parallel query by default.

Change max_parallel_degree default from 0 to 2.  It is possible that
this is not a good idea, or that we should go with 1 worker rather
than 2, but we won't find out without trying it.  Along the way,
reword the documentation for max_parallel_degree a little bit to
hopefully make it more clear.

Discussion: 20160420174631.3qjjhpwsvvx5bau5@alap3.anarazel.de

doc/src/sgml/config.sgml

@@ -2005,10 +2005,14 @@ include_dir 'conf.d'
        </term>
        <listitem>
         <para>
-         Sets the maximum degree of parallelism for an individual parallel
-         operation.  Note that the requested number of workers may not actually
-         be available at runtime.  Parallel workers are taken from the pool
-         of processes established by <xref linkend="guc-max-worker-processes">.
+         Sets the maximum number of workers that can be started for an
+         individual parallel operation.  Parallel workers are taken from the
+         pool of processes established by
+         <xref linkend="guc-max-worker-processes">.  Note that the requested
+         number of workers may not actually be available at runtime.  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
+         disables parallel query.
         </para>
        </listitem>
       </varlistentry>

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_degree = 0;
+int			max_parallel_degree = 2;
 
 bool		enable_seqscan = true;
 bool		enable_indexscan = true;

src/backend/utils/misc/guc.c

@@ -2662,7 +2662,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_parallel_degree,
-		0, 0, MAX_BACKENDS,
+		2, 0, MAX_BACKENDS,
 		NULL, NULL, NULL
 	},
 

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

@@ -167,7 +167,7 @@
 
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching
 #max_worker_processes = 8
-#max_parallel_degree = 0		# max number of worker processes per node
+#max_parallel_degree = 2		# max number of worker processes per node
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 									# (change requires restart)
 #backend_flush_after = 0		# 0 disables,