Tweak the default behavior of psql's \dconfig.

\dconfig without an argument originally printed all parameters,
but it seems more useful to print only those parameters with
non-default settings.  You can easily get the show-everything
behavior with "\dconfig *", but that output is unwieldy and
seems unlikely to be wanted very often.

Per suggestion from Christoph Berg.

Discussion: https://postgr.es/m/YlFQLzlPi4QD0wSi@msg.df7cb.de

doc/src/sgml/ref/psql-ref.sgml

@@ -129,7 +129,7 @@ echo '\x \\ SELECT * FROM foo;' | psql
        for more details about how the server handles multi-query strings.)
       </para>
       <para>
-       If having several commands executed in one transaction is not desired, 
+       If having several commands executed in one transaction is not desired,
        use repeated <option>-c</option> commands or feed multiple commands to
        <application>psql</application>'s standard input,
        either using <application>echo</application> as illustrated above, or
@@ -1385,9 +1385,11 @@ testdb=&gt;
         <listitem>
         <para>
         Lists server configuration parameters and their values.
-        If <replaceable class="parameter">pattern</replaceable>
-        is specified, only parameters whose names match the pattern are
-        listed.
+        If <replaceable class="parameter">pattern</replaceable> is specified,
+        only parameters whose names match the pattern are listed.  Without
+        a <replaceable class="parameter">pattern</replaceable>, only
+        parameters that are set to non-default values are listed.
+        (Use <literal>\dconfig *</literal> to see all parameters.)
         If <literal>+</literal> is appended to the command name, each
         parameter is listed with its data type, context in which the
         parameter can be set, and access privileges (if non-default access

src/bin/psql/describe.c

@@ -4404,10 +4404,13 @@ describeConfigurationParameters(const char *pattern, bool verbose,
 							 "  LEFT JOIN pg_catalog.pg_parameter_acl p\n"
 							 "  ON pg_catalog.lower(s.name) = p.parname\n");
 
-	processSQLNamePattern(pset.db, &buf, pattern,
-						  false, false,
-						  NULL, "pg_catalog.lower(s.name)", NULL,
-						  NULL);
+	if (pattern)
+		processSQLNamePattern(pset.db, &buf, pattern,
+							  false, false,
+							  NULL, "pg_catalog.lower(s.name)", NULL,
+							  NULL);
+	else
+		appendPQExpBufferStr(&buf, "WHERE s.source <> 'default'\n");
 
 	appendPQExpBufferStr(&buf, "ORDER BY 1;");
 
@@ -4417,7 +4420,10 @@ describeConfigurationParameters(const char *pattern, bool verbose,
 		return false;
 
 	myopt.nullPrint = NULL;
-	myopt.title = _("List of configuration parameters");
+	if (pattern)
+		myopt.title = _("List of configuration parameters");
+	else
+		myopt.title = _("List of non-default configuration parameters");
 	myopt.translate_header = true;
 
 	printQuery(res, &myopt, pset.queryFout, false, pset.logfile);