Make some more improvements to parallel query documentation.

Many places that mentioned only Gather should also mention Gather
Merge, or should be phrased in a more neutral way.  Be more clear
about the fact that max_parallel_workers_per_gather affects the number
of workers the planner may want to use.  Fix a typo.  Explain how
Gather Merge works.  Adjust wording around parallel scans to be a bit
more clear.  Adjust wording around parallel-restricted operations for
the fact that uncorrelated subplans are no longer restricted.

Patch by me, reviewed by Erik Rijkers

Discussion: http://postgr.es/m/CA+TgmoZsTjgVGn=ei5ht-1qGFKy_m1VgB3d8+Rg304hz91N5ww@mail.gmail.com

doc/src/sgml/config.sgml

@@ -2050,8 +2050,8 @@ include_dir 'conf.d'
        <listitem>
         <para>
          Sets the maximum number of workers that can be started by a single
-         <literal>Gather</literal> node.  Parallel workers are taken from the
+         <literal>Gather</literal> or <literal>Gather Merge</literal> node.
-         pool of processes established by
+         Parallel workers are taken from the pool of processes established by
          <xref linkend="guc-max-worker-processes">, limited by
          <xref linkend="guc-max-parallel-workers">.  Note that the requested
          number of workers may not actually be available at run time.  If this

doc/src/sgml/parallel.sgml

@@ -28,7 +28,8 @@
    <para>
     When the optimizer determines that parallel query is the fastest execution
     strategy for a particular query, it will create a query plan which includes
-    a <firstterm>Gather node</firstterm>.  Here is a simple example:
+    a <firstterm>Gather</firstterm> or <firstterm>Gather Merge</firstterm>
+    node.  Here is a simple example:
 
 <screen>
 EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
@@ -43,15 +44,16 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
    </para>
 
    <para>
-    In all cases, the <literal>Gather</literal> node will have exactly one
+    In all cases, the <literal>Gather</literal> or
+    <literal>Gather Merge</literal> node will have exactly one
     child plan, which is the portion of the plan that will be executed in
-    parallel.  If the <literal>Gather</> node is at the very top of the plan
+    parallel.  If the <literal>Gather</> or <literal>Gather Merge</> node is
-    tree, then the entire query will execute in parallel.  If it is somewhere
+    at the very top of the plan tree, then the entire query will execute in
-    else in the plan tree, then only the portion of the plan below it will run
+    parallel.  If it is somewhere else in the plan tree, then only the portion
-    in parallel.  In the example above, the query accesses only one table, so
+    of the plan below it will run in parallel.  In the example above, the
-    there is only one plan node other than the <literal>Gather</> node itself;
+    query accesses only one table, so there is only one plan node other than
-    since that plan node is a child of the <literal>Gather</> node, it will
+    the <literal>Gather</> node itself; since that plan node is a child of the
-    run in parallel.
+    <literal>Gather</> node, it will run in parallel.
    </para>
 
    <para>
@@ -60,34 +62,46 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     during query execution, the process which is implementing the user's
     session will request a number of <link linkend="bgworker">background
     worker processes</link> equal to the number
-    of workers chosen by the planner.  The total number of background
+    of workers chosen by the planner.  The number of background workers that
-    workers that can exist at any one time is limited by both
+    the planner will consider using is limited to at most
+    <xref linkend="guc-max-parallel-workers-per-gather">.  The total number
+    of background workers that can exist at any one time is limited by both
     <xref linkend="guc-max-worker-processes"> and
-    <xref linkend="guc-max-parallel-workers">, so it is possible for a
+    <xref linkend="guc-max-parallel-workers">.  Therefore, it is possible for a
     parallel query to run with fewer workers than planned, or even with
     no workers at all.  The optimal plan may depend on the number of workers
     that are available, so this can result in poor query performance.  If this
-    occurrence is frequent, considering increasing
+    occurrence is frequent, consider increasing
     <varname>max_worker_processes</> and <varname>max_parallel_workers</>
     so that more workers can be run simultaneously or alternatively reducing
-    <xref linkend="guc-max-parallel-workers-per-gather"> so that the planner
+    <varname>max_parallel_workers_per_gather</varname> so that the planner
     requests fewer workers.
    </para>
 
    <para>
     Every background worker process which is successfully started for a given
-    parallel query will execute the portion of the plan below
+    parallel query will execute the parallel portion of the plan.  The leader
-    the <literal>Gather</> node.  The leader will also execute that portion
+    will also execute that portion of the plan, but it has an additional
-    of the plan, but it has an additional responsibility: it must also read
+    responsibility: it must also read all of the tuples generated by the
-    all of the tuples generated by the workers.  When the parallel portion of
+    workers.  When the parallel portion of the plan generates only a small
-    the plan generates only a small number of tuples, the leader will often
+    number of tuples, the leader will often behave very much like an additional
-    behave very much like an additional worker, speeding up query execution.
+    worker, speeding up query execution.  Conversely, when the parallel portion
-    Conversely, when the parallel portion of the plan generates a large number
+    of the plan generates a large number of tuples, the leader may be almost
-    of tuples, the leader may be almost entirely occupied with reading the
+    entirely occupied with reading the tuples generated by the workers and
-    tuples generated by the workers and performing any further processing
+    performing any further processing steps which are required by plan nodes
-    steps which are required by plan nodes above the level of the
+    above the level of the <literal>Gather</literal> node or
-    <literal>Gather</literal> node.  In such cases, the leader will do very
+    <literal>Gather Merge</literal> node.  In such cases, the leader will
-    little of the work of executing the parallel portion of the plan.
+    do very little of the work of executing the parallel portion of the plan.
+   </para>
+
+   <para>
+    When the node at the top of the parallel portion of the plan is
+    <literal>Gather Merge</> rather than <literal>Gather</>, it indicates that
+    each process executing the parallel portion of the plan is producing
+    tuples in sorted order, and that the leader is performing an
+    order-preserving merge.  In contrast, <literal>Gather</> reads tuples
+    from the workers in whatever order is convenient, destroying any sort
+    order that may have existed.
    </para>   
  </sect1>
 
@@ -221,9 +235,9 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
         send such a message, this can only occur when using a client that
         does not rely on libpq.  If this is a frequent
         occurrence, it may be a good idea to set
-        <xref linkend="guc-max-parallel-workers-per-gather"> in sessions
+        <xref linkend="guc-max-parallel-workers-per-gather"> to zero in
-        where it is likely, so as to avoid generating query plans that may
+        sessions where it is likely, so as to avoid generating query plans
-        be suboptimal when run serially.
+        that may be suboptimal when run serially.
       </para>
     </listitem>
 
@@ -262,6 +276,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     so that each process which executes the plan will generate only a
     subset of the output rows in such a way that each required output row
     is guaranteed to be generated by exactly one of the cooperating processes.
+    Generally, this means that the scan on the driving table of the query
+    must be a parallel-aware scan.
   </para>
 
  <sect2 id="parallel-scans">
@@ -302,9 +318,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     </listitem>
   </itemizedlist>
 
-    Only the scan types listed above may be used for a scan on the driving
+    Other scan types, such as scans of non-btree indexes, may support
-    table within a parallel plan.  Other scan types, such as parallel scans of
+    parallel scans in the future.
-    non-btree indexes, may be supported in the future.
   </para>
  </sect2>
 
@@ -343,10 +358,10 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     the query performs an aggregation step, producing a partial result for
     each group of which that process is aware.  This is reflected in the plan
     as a <literal>Partial Aggregate</> node.  Second, the partial results are
-    transferred to the leader via the <literal>Gather</> node.  Finally, the
+    transferred to the leader via <literal>Gather</> or <literal>Gather
-    leader re-aggregates the results across all workers in order to produce
+    Merge</>.  Finally, the leader re-aggregates the results across all
-    the final result.  This is reflected in the plan as a
+    workers in order to produce the final result.  This is reflected in the
-    <literal>Finalize Aggregate</> node.
+    plan as a <literal>Finalize Aggregate</> node.
   </para>
   
   <para>
@@ -416,8 +431,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     operation is one which cannot be performed in a parallel worker, but which
     can be performed in the leader while parallel query is in use.  Therefore,
     parallel restricted operations can never occur below a <literal>Gather</>
-    node, but can occur elsewhere in a plan which contains a
+    or <literal>Gather Merge</> node, but can occur elsewhere in a plan which
-    <literal>Gather</> node.  A parallel unsafe operation is one which cannot
+    contains such a node.  A parallel unsafe operation is one which cannot
     be performed while parallel query is in use, not even in the leader.
     When a query contains anything which is parallel unsafe, parallel query
     is completely disabled for that query.
@@ -449,7 +464,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
 
     <listitem>
       <para>
-        Access to an <literal>InitPlan</> or <literal>SubPlan</>.
+        Access to an <literal>InitPlan</> or correlated <literal>SubPlan</>.
       </para>
     </listitem>
   </itemizedlist>
@@ -514,8 +529,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     parallel-restricted functions or aggregates involved in the query in
     order to obtain a superior plan.  So, for example, if a <literal>WHERE</>
     clause applied to a particular table is parallel restricted, the query
-    planner will not consider placing the scan of that table below a
+    planner will not consider performing a scan of that table in the parallel
-    <literal>Gather</> node.  In some cases, it would be
+    portion of a plan.  In some cases, it would be
     possible (and perhaps even efficient) to include the scan of that table in
     the parallel portion of the query and defer the evaluation of the
     <literal>WHERE</> clause so that it happens above the <literal>Gather</>