Add SETTINGS option to EXPLAIN, to print modified settings.

Query planning is affected by a number of configuration options, and it
may be crucial to know which of those options were set to non-default
values.  With this patch you can say EXPLAIN (SETTINGS ON) to include
that information in the query plan.  Only options affecting planning,
with values different from the built-in default are printed.

This patch also adds auto_explain.log_settings option, providing the
same capability in auto_explain module.

Author: Tomas Vondra
Reviewed-by: Rafia Sabih, John Naylor
Discussion: https://postgr.es/m/e1791b4c-df9c-be02-edc5-7c8874944be0@2ndquadrant.com

Author: tvondra

contrib/auto_explain/auto_explain.c

@@ -28,6 +28,7 @@ static bool auto_explain_log_verbose = false;
 static bool auto_explain_log_buffers = false;
 static bool auto_explain_log_triggers = false;
 static bool auto_explain_log_timing = true;
+static bool auto_explain_log_settings = false;
 static int	auto_explain_log_format = EXPLAIN_FORMAT_TEXT;
 static int	auto_explain_log_level = LOG;
 static bool auto_explain_log_nested_statements = false;
@@ -112,6 +113,17 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
+	DefineCustomBoolVariable("auto_explain.log_settings",
+							 "Log modified configuration parameters affecting query planning.",
+							 NULL,
+							 &auto_explain_log_settings,
+							 false,
+							 PGC_SUSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+
 	DefineCustomBoolVariable("auto_explain.log_verbose",
 							 "Use EXPLAIN VERBOSE for plan logging.",
 							 NULL,
@@ -356,6 +368,7 @@ explain_ExecutorEnd(QueryDesc *queryDesc)
 			es->timing = (es->analyze && auto_explain_log_timing);
 			es->summary = es->analyze;
 			es->format = auto_explain_log_format;
+			es->settings = auto_explain_log_settings;
 
 			ExplainBeginOutput(es);
 			ExplainQueryText(es, queryDesc);

doc/src/sgml/auto-explain.sgml

@@ -169,6 +169,24 @@ LOAD 'auto_explain';
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term>
+     <varname>auto_explain.log_settings</varname> (<type>boolean</type>)
+     <indexterm>
+      <primary><varname>auto_explain.log_settings</varname> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      <varname>auto_explain.log_settings</varname> controls whether information
+      about modified configuration options are printed when execution plan is logged.
+      Only options affecting query planning with value different from the built-in
+      default value are included in the output.  This parameter is off by default.
+      Only superusers can change this setting.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term>
      <varname>auto_explain.log_format</varname> (<type>enum</type>)

doc/src/sgml/ref/explain.sgml

@@ -39,6 +39,7 @@ EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replac
     ANALYZE [ <replaceable class="parameter">boolean</replaceable> ]
     VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
     COSTS [ <replaceable class="parameter">boolean</replaceable> ]
+    SETTINGS [ <replaceable class="parameter">boolean</replaceable> ]
     BUFFERS [ <replaceable class="parameter">boolean</replaceable> ]
     TIMING [ <replaceable class="parameter">boolean</replaceable> ]
     SUMMARY [ <replaceable class="parameter">boolean</replaceable> ]
@@ -152,6 +153,17 @@ ROLLBACK;
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>SETTINGS</literal></term>
+    <listitem>
+     <para>
+      Include information on configuration parameters.  Specifically, include
+      options affecting query planning with value different from the built-in
+      default value.  This parameter defaults to <literal>FALSE</literal>.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>BUFFERS</literal></term>
     <listitem>

src/backend/commands/explain.c

@@ -29,6 +29,7 @@
 #include "storage/bufmgr.h"
 #include "tcop/tcopprot.h"
 #include "utils/builtins.h"
+#include "utils/guc_tables.h"
 #include "utils/json.h"
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
@@ -162,6 +163,8 @@ ExplainQuery(ParseState *pstate, ExplainStmt *stmt, const char *queryString,
 			es->costs = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "buffers") == 0)
 			es->buffers = defGetBoolean(opt);
+		else if (strcmp(opt->defname, "settings") == 0)
+			es->settings = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "timing") == 0)
 		{
 			timing_set = true;
@@ -596,6 +599,73 @@ ExplainOnePlan(PlannedStmt *plannedstmt, IntoClause *into, ExplainState *es,
 	ExplainCloseGroup("Query", NULL, true, es);
 }
 
+/*
+ * ExplainPrintSettings -
+ *    Print summary of modified settings affecting query planning.
+ */
+static void
+ExplainPrintSettings(ExplainState *es)
+{
+	int		num;
+	struct config_generic **gucs;
+
+	/* bail out if information about settings not requested */
+	if (!es->settings)
+		return;
+
+	/* request an array of relevant settings */
+	gucs = get_explain_guc_options(&num);
+
+	/* also bail out of there are no options */
+	if (!num)
+		return;
+
+	if (es->format != EXPLAIN_FORMAT_TEXT)
+	{
+		int		i;
+
+		ExplainOpenGroup("Settings", "Settings", true, es);
+
+		for (i = 0; i < num; i++)
+		{
+			char *setting;
+			struct config_generic *conf = gucs[i];
+
+			setting = GetConfigOptionByName(conf->name, NULL, true);
+
+			ExplainPropertyText(conf->name, setting, es);
+		}
+
+		ExplainCloseGroup("Settings", "Settings", true, es);
+	}
+	else
+	{
+		int		i;
+		StringInfoData	str;
+
+		initStringInfo(&str);
+
+		for (i = 0; i < num; i++)
+		{
+			char *setting;
+			struct config_generic *conf = gucs[i];
+
+			if (i > 0)
+				appendStringInfoString(&str, ", ");
+
+			setting = GetConfigOptionByName(conf->name, NULL, true);
+
+			if (setting)
+				appendStringInfo(&str, "%s = '%s'", conf->name, setting);
+			else
+				appendStringInfo(&str, "%s = NULL", conf->name);
+		}
+
+		if (num > 0)
+			ExplainPropertyText("Settings", str.data, es);
+	}
+}
+
 /*
  * ExplainPrintPlan -
  *	  convert a QueryDesc's plan tree to text and append it to es->str
@@ -633,6 +703,12 @@ ExplainPrintPlan(ExplainState *es, QueryDesc *queryDesc)
 	if (IsA(ps, GatherState) &&((Gather *) ps->plan)->invisible)
 		ps = outerPlanState(ps);
 	ExplainNode(ps, NIL, NULL, NULL, es);
+
+	/*
+	 * If requested, include information about GUC parameters with values
+	 * that don't match the built-in defaults.
+	 */
+	ExplainPrintSettings(es);
 }
 
 /*