Eliminate "parallel degree" terminology.

This terminology provoked widespread complaints.  So, instead, rename
the GUC max_parallel_degree to max_parallel_workers_per_gather
(leaving room for a possible future GUC max_parallel_workers that acts
as a system-wide limit), and rename the parallel_degree reloption to
parallel_workers.  Rename structure members to match.

These changes create a dump/restore hazard for users of PostgreSQL
9.6beta1 who have set the reloption (or applied the GUC using ALTER
USER or ALTER DATABASE).

Author: robertmhaas

doc/src/sgml/config.sgml

@@ -1998,16 +1998,16 @@ include_dir 'conf.d'
        </listitem>
       </varlistentry>
 
-      <varlistentry id="guc-max-parallel-degree" xreflabel="max_parallel_degree">
-       <term><varname>max_parallel_degree</varname> (<type>integer</type>)
+      <varlistentry id="guc-max-parallel-workers-per-gather" xreflabel="max_parallel_workers_per_gather">
+       <term><varname>max_parallel_workers_per_gather</varname> (<type>integer</type>)
        <indexterm>
-        <primary><varname>max_parallel_degree</> configuration parameter</primary>
+        <primary><varname>max_parallel_workers_per_gather</> configuration parameter</primary>
        </indexterm>
        </term>
        <listitem>
         <para>
-         Sets the maximum number of workers that can be started for an
-         individual parallel operation.  Parallel workers are taken from the
+         Sets the maximum number of workers that can be started by a single
+         <literal>Gather</literal> node.  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

doc/src/sgml/ref/create_table.sgml

@@ -909,14 +909,14 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
    </varlistentry>
 
    <varlistentry>
-    <term><literal>parallel_degree</> (<type>integer</>)</term>
+    <term><literal>parallel_workers</> (<type>integer</>)</term>
     <listitem>
      <para>
-      The parallel degree for a table is the number of workers that should
-      be used to assist a parallel scan of that table.  If not set, the
-      system will determine a value based on the relation size.  The actual
-      number of workers chosen by the planner may be less, for example due to
-      the setting of <xref linkend="guc-max-parallel-degree">.
+      This sets the number of workers that should be used to assist a parallel
+      scan of this table.  If not set, the system will determine a value based
+      on the relation size.  The actual number of workers chosen by the planner
+      may be less, for example due to
+      the setting of <xref linkend="guc-max-worker-processes">.
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/release-9.6.sgml

@@ -153,7 +153,7 @@
        <para>
         Use of parallel query execution can be controlled through the new
         configuration parameters
-        <xref linkend="guc-max-parallel-degree">,
+        <xref linkend="guc-max-parallel-workers-per-gather">,
         <xref linkend="guc-force-parallel-mode">,
         <xref linkend="guc-parallel-setup-cost">, and
         <xref linkend="guc-parallel-tuple-cost">.

src/backend/access/common/reloptions.c

@@ -270,7 +270,7 @@ static relopt_int intRelOpts[] =
 	},
 	{
 		{
-			"parallel_degree",
+			"parallel_workers",
 			"Number of parallel processes that can be used per executor node for this relation.",
 			RELOPT_KIND_HEAP,
 			AccessExclusiveLock
@@ -1301,8 +1301,8 @@ default_reloptions(Datum reloptions, bool validate, relopt_kind kind)
 		offsetof(StdRdOptions, autovacuum) +offsetof(AutoVacOpts, analyze_scale_factor)},
 		{"user_catalog_table", RELOPT_TYPE_BOOL,
 		offsetof(StdRdOptions, user_catalog_table)},
-		{"parallel_degree", RELOPT_TYPE_INT,
-		offsetof(StdRdOptions, parallel_degree)}
+		{"parallel_workers", RELOPT_TYPE_INT,
+		offsetof(StdRdOptions, parallel_workers)}
 	};
 
 	options = parseRelOptions(reloptions, validate, kind, &numoptions);

src/backend/nodes/outfuncs.c

@@ -1609,7 +1609,7 @@ _outPathInfo(StringInfo str, const Path *node)
 		_outBitmapset(str, NULL);
 	WRITE_BOOL_FIELD(parallel_aware);
 	WRITE_BOOL_FIELD(parallel_safe);
-	WRITE_INT_FIELD(parallel_degree);
+	WRITE_INT_FIELD(parallel_workers);
 	WRITE_FLOAT_FIELD(rows, "%.0f");
 	WRITE_FLOAT_FIELD(startup_cost, "%.2f");
 	WRITE_FLOAT_FIELD(total_cost, "%.2f");

src/backend/optimizer/path/allpaths.c

@@ -669,26 +669,26 @@ set_plain_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 static void
 create_plain_partial_paths(PlannerInfo *root, RelOptInfo *rel)
 {
-	int			parallel_degree = 1;
+	int			parallel_workers = 1;
 
 	/*
-	 * If the user has set the parallel_degree reloption, we decide what to do
+	 * If the user has set the parallel_workers reloption, we decide what to do
 	 * based on the value of that option.  Otherwise, we estimate a value.
 	 */
-	if (rel->rel_parallel_degree != -1)
+	if (rel->rel_parallel_workers != -1)
 	{
 		/*
-		 * If parallel_degree = 0 is set for this relation, bail out.  The
+		 * If parallel_workers = 0 is set for this relation, bail out.  The
 		 * user does not want a parallel path for this relation.
 		 */
-		if (rel->rel_parallel_degree == 0)
+		if (rel->rel_parallel_workers == 0)
 			return;
 
 		/*
-		 * Use the table parallel_degree, but don't go further than
-		 * max_parallel_degree.
+		 * Use the table parallel_workers, but don't go further than
+		 * max_parallel_workers_per_gather.
 		 */
-		parallel_degree = Min(rel->rel_parallel_degree, max_parallel_degree);
+		parallel_workers = Min(rel->rel_parallel_workers, max_parallel_workers_per_gather);
 	}
 	else
 	{
@@ -711,17 +711,17 @@ create_plain_partial_paths(PlannerInfo *root, RelOptInfo *rel)
 		 * sophisticated, but we need something here for now.
 		 */
 		while (rel->pages > parallel_threshold * 3 &&
-			   parallel_degree < max_parallel_degree)
+			   parallel_workers < max_parallel_workers_per_gather)
 		{
-			parallel_degree++;
+			parallel_workers++;
 			parallel_threshold *= 3;
 			if (parallel_threshold >= PG_INT32_MAX / 3)
 				break;
 		}
 	}
 
 	/* Add an unordered partial path based on a parallel sequential scan. */
-	add_partial_path(rel, create_seqscan_path(root, rel, NULL, parallel_degree));
+	add_partial_path(rel, create_seqscan_path(root, rel, NULL, parallel_workers));
 }
 
 /*
@@ -1242,25 +1242,25 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	{
 		AppendPath *appendpath;
 		ListCell   *lc;
-		int			parallel_degree = 0;
+		int			parallel_workers = 0;
 
 		/*
-		 * Decide what parallel degree to request for this append path.  For
-		 * now, we just use the maximum parallel degree of any member.  It
+		 * Decide on the numebr of workers to request for this append path.  For
+		 * now, we just use the maximum value from among the members.  It
 		 * might be useful to use a higher number if the Append node were
 		 * smart enough to spread out the workers, but it currently isn't.
 		 */
 		foreach(lc, partial_subpaths)
 		{
 			Path	   *path = lfirst(lc);
 
-			parallel_degree = Max(parallel_degree, path->parallel_degree);
+			parallel_workers = Max(parallel_workers, path->parallel_workers);
 		}
-		Assert(parallel_degree > 0);
+		Assert(parallel_workers > 0);
 
 		/* Generate a partial append path. */
 		appendpath = create_append_path(rel, partial_subpaths, NULL,
-										parallel_degree);
+										parallel_workers);
 		add_partial_path(rel, (Path *) appendpath);
 	}
 

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 = 2;
+int			max_parallel_workers_per_gather = 2;
 
 bool		enable_seqscan = true;
 bool		enable_indexscan = true;
@@ -229,9 +229,9 @@ cost_seqscan(Path *path, PlannerInfo *root,
 	cpu_run_cost += path->pathtarget->cost.per_tuple * path->rows;
 
 	/* Adjust costing for parallelism, if used. */
-	if (path->parallel_degree > 0)
+	if (path->parallel_workers > 0)
 	{
-		double		parallel_divisor = path->parallel_degree;
+		double		parallel_divisor = path->parallel_workers;
 		double		leader_contribution;
 
 		/*
@@ -245,7 +245,7 @@ cost_seqscan(Path *path, PlannerInfo *root,
 		 * estimate that the leader spends 30% of its time servicing each
 		 * worker, and the remainder executing the parallel plan.
 		 */
-		leader_contribution = 1.0 - (0.3 * path->parallel_degree);
+		leader_contribution = 1.0 - (0.3 * path->parallel_workers);
 		if (leader_contribution > 0)
 			parallel_divisor += leader_contribution;
 

src/backend/optimizer/plan/createplan.c

@@ -1394,7 +1394,7 @@ create_gather_plan(PlannerInfo *root, GatherPath *best_path)
 
 	gather_plan = make_gather(tlist,
 							  NIL,
-							  best_path->path.parallel_degree,
+							  best_path->path.parallel_workers,
 							  best_path->single_copy,
 							  subplan);
 

src/backend/optimizer/plan/planner.c

@@ -245,7 +245,7 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 	glob->parallelModeOK = (cursorOptions & CURSOR_OPT_PARALLEL_OK) != 0 &&
 		IsUnderPostmaster && dynamic_shared_memory_type != DSM_IMPL_NONE &&
 		parse->commandType == CMD_SELECT && !parse->hasModifyingCTE &&
-		parse->utilityStmt == NULL && max_parallel_degree > 0 &&
+		parse->utilityStmt == NULL && max_parallel_workers_per_gather > 0 &&
 		!IsParallelWorker() && !IsolationIsSerializable() &&
 		!has_parallel_hazard((Node *) parse, true);
 
@@ -3622,7 +3622,7 @@ create_grouping_paths(PlannerInfo *root,
 		if (grouped_rel->partial_pathlist)
 		{
 			Path   *path = (Path *) linitial(grouped_rel->partial_pathlist);
-			double total_groups = path->rows * path->parallel_degree;
+			double total_groups = path->rows * path->parallel_workers;
 
 			path = (Path *) create_gather_path(root,
 											   grouped_rel,
@@ -3717,7 +3717,7 @@ create_grouping_paths(PlannerInfo *root,
 
 			if (hashaggtablesize < work_mem * 1024L)
 			{
-				double total_groups = path->rows * path->parallel_degree;
+				double total_groups = path->rows * path->parallel_workers;
 
 				path = (Path *) create_gather_path(root,
 												   grouped_rel,