Assorted documentation improvements for max_parallel_workers.

Commit b460f5d6693103076dc554aa7cbb96e1e53074f9 overlooked a few bits
of documentation that seem like they should mention the new setting.

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index b917f9578ab37c951e93d8c17c695502bd57c780..0fc4e57d900b08c239be6b8f98626b59063b3ff6 100644 (file)
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -1990,6 +1990,12 @@ include_dir 'conf.d'
          same or higher value than on the master server. Otherwise, queries
          will not be allowed in the standby server.
         </para>
+
+        <para>
+         When changing this value, consider also adjusting
+         <xref linkend="guc-max-parallel-workers"> and
+         <xref linkend="guc-max-parallel-workers-per-gather">.
+        </para>
        </listitem>
       </varlistentry>
 
@@ -2047,6 +2053,10 @@ include_dir 'conf.d'
          parallel queries.  The default value is 8.  When increasing or
          decreasing this value, consider also adjusting
          <xref linkend="guc-max-parallel-workers-per-gather">.
+         Also, note that a setting for this value which is higher than
+         <xref linkend="guc-max-worker-processes"> will have no effect,
+         since parallel workers are taken from the pool of worker processes
+         established by that setting.
         </para>
        </listitem>
       </varlistentry>

diff --git a/doc/src/sgml/parallel.sgml b/doc/src/sgml/parallel.sgml
index 38a040ef75ec7f9ede10f6a3823dad0ea829191b..f39c21a4550329257c778567c0a3f168e28f447c 100644 (file)
--- a/doc/src/sgml/parallel.sgml
+++ b/doc/src/sgml/parallel.sgml
@@ -61,14 +61,15 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
     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
-    workers that can exist at any one time is limited by
-    <xref linkend="guc-max-worker-processes">, so it is possible for a
+    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
     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
-    <varname>max_worker_processes</> so that more workers can be run
-    simultaneously or alternatively reducing
+    <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
     requests fewer workers.
    </para>
@@ -203,6 +204,14 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
       </para>
     </listitem>
 
+    <listitem>
+      <para> 
+        No background workers can be obtained because of the limitation that
+        the total number of background workers launched for purposes of
+        parallel query cannot exceed <xref linkend="guc-max-parallel-workers">.
+      </para>
+    </listitem>
+
     <listitem>
       <para> 
         The client sends an Execute message with a non-zero fetch count.