[CF 5751] v1 - Disable parallel query by default

This branch was automatically generated by a robot using patches from an
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https://commitfest.postgresql.org/patch/5751

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Patch(es): https://www.postgresql.org/message-id/a5916f83-de79-4a40-933a-fb0d9ba2f5a0@app.fastmail.com
Author(s): Scott Mead

Author: Commitfest Bot

doc/src/sgml/config.sgml

@@ -2855,8 +2855,8 @@ include_dir 'conf.d'
          <xref linkend="guc-max-parallel-workers"/>.  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
-         disables parallel query execution.
+         be inefficient.  The default value is 0.  Setting this value to 2 or
+         higher enables parallel query execution.
         </para>
 
         <para>

src/backend/optimizer/path/costsize.c

@@ -140,7 +140,7 @@ 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_tables.c

@@ -3621,7 +3621,7 @@ struct config_int ConfigureNamesInt[] =
 			GUC_EXPLAIN
 		},
 		&max_parallel_workers_per_gather,
-		2, 0, MAX_PARALLEL_WORKER_LIMIT,
+		0, 0, MAX_PARALLEL_WORKER_LIMIT,
 		NULL, NULL, NULL
 	},
 

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

@@ -215,7 +215,7 @@
 # - Worker Processes -
 
 #max_worker_processes = 8		# (change requires restart)
-#max_parallel_workers_per_gather = 2	# limited by max_parallel_workers
+#max_parallel_workers_per_gather = 0	# limited by max_parallel_workers
 #max_parallel_maintenance_workers = 2	# limited by max_parallel_workers
 #max_parallel_workers = 8		# number of max_worker_processes that
 					# can be used in parallel operations