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

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);
 }
 
 /*

src/backend/utils/misc/guc.c

@@ -882,7 +882,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_seqscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of sequential-scan plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_seqscan,
 		true,
@@ -891,7 +892,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_indexscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of index-scan plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_indexscan,
 		true,
@@ -900,7 +902,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_indexonlyscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of index-only-scan plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_indexonlyscan,
 		true,
@@ -909,7 +912,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_bitmapscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of bitmap-scan plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_bitmapscan,
 		true,
@@ -918,7 +922,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_tidscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of TID scan plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_tidscan,
 		true,
@@ -927,7 +932,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_sort", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of explicit sort steps."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_sort,
 		true,
@@ -936,7 +942,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_hashagg", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of hashed aggregation plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_hashagg,
 		true,
@@ -945,7 +952,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_material", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of materialization."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_material,
 		true,
@@ -954,7 +962,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_nestloop", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of nested-loop join plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_nestloop,
 		true,
@@ -963,7 +972,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_mergejoin", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of merge join plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_mergejoin,
 		true,
@@ -972,7 +982,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_hashjoin", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of hash join plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_hashjoin,
 		true,
@@ -981,7 +992,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_gathermerge", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of gather merge plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_gathermerge,
 		true,
@@ -990,7 +1002,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_partitionwise_join", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables partitionwise join."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_partitionwise_join,
 		false,
@@ -999,7 +1012,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_partitionwise_aggregate", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables partitionwise aggregation and grouping."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_partitionwise_aggregate,
 		false,
@@ -1008,7 +1022,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_parallel_append", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of parallel append plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_parallel_append,
 		true,
@@ -1017,7 +1032,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"enable_parallel_hash", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of parallel hash plans."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&enable_parallel_hash,
 		true,
@@ -1028,7 +1044,8 @@ static struct config_bool ConfigureNamesBool[] =
 			gettext_noop("Enable plan-time and run-time partition pruning."),
 			gettext_noop("Allows the query planner and executor to compare partition "
 						 "bounds to conditions in the query to determine which "
-						 "partitions must be scanned.")
+						 "partitions must be scanned."),
+			GUC_EXPLAIN
 		},
 		&enable_partition_pruning,
 		true,
@@ -1038,7 +1055,8 @@ static struct config_bool ConfigureNamesBool[] =
 		{"geqo", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("Enables genetic query optimization."),
 			gettext_noop("This algorithm attempts to do planning without "
-						 "exhaustive searching.")
+						 "exhaustive searching."),
+			GUC_EXPLAIN
 		},
 		&enable_geqo,
 		true,
@@ -1646,7 +1664,7 @@ static struct config_bool ConfigureNamesBool[] =
 			"optimize_bounded_sort", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enable bounded sorting using heap sort."),
 			NULL,
-			GUC_NOT_IN_SAMPLE
+			GUC_NOT_IN_SAMPLE | GUC_EXPLAIN
 		},
 		&optimize_bounded_sort,
 		true,
@@ -1837,7 +1855,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"parallel_leader_participation", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
 			gettext_noop("Controls whether Gather and Gather Merge also run subplans."),
-			gettext_noop("Should gather nodes also run subplans, or just gather tuples?")
+			gettext_noop("Should gather nodes also run subplans, or just gather tuples?"),
+			GUC_EXPLAIN
 		},
 		&parallel_leader_participation,
 		true,
@@ -1847,7 +1866,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"jit", PGC_USERSET, QUERY_TUNING_OTHER,
 			gettext_noop("Allow JIT compilation."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&jit_enabled,
 		true,
@@ -1976,7 +1996,8 @@ static struct config_int ConfigureNamesInt[] =
 						 "are not collapsed."),
 			gettext_noop("The planner will merge subqueries into upper "
 						 "queries if the resulting FROM list would have no more than "
-						 "this many items.")
+						 "this many items."),
+			GUC_EXPLAIN
 		},
 		&from_collapse_limit,
 		8, 1, INT_MAX,
@@ -1988,7 +2009,8 @@ static struct config_int ConfigureNamesInt[] =
 						 "constructs are not flattened."),
 			gettext_noop("The planner will flatten explicit JOIN "
 						 "constructs into lists of FROM items whenever a "
-						 "list of no more than this many items would result.")
+						 "list of no more than this many items would result."),
+			GUC_EXPLAIN
 		},
 		&join_collapse_limit,
 		8, 1, INT_MAX,
@@ -1997,7 +2019,8 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"geqo_threshold", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("Sets the threshold of FROM items beyond which GEQO is used."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&geqo_threshold,
 		12, 2, INT_MAX,
@@ -2006,7 +2029,8 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"geqo_effort", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("GEQO: effort is used to set the default for other GEQO parameters."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&Geqo_effort,
 		DEFAULT_GEQO_EFFORT, MIN_GEQO_EFFORT, MAX_GEQO_EFFORT,
@@ -2015,7 +2039,8 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"geqo_pool_size", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("GEQO: number of individuals in the population."),
-			gettext_noop("Zero selects a suitable default value.")
+			gettext_noop("Zero selects a suitable default value."),
+			GUC_EXPLAIN
 		},
 		&Geqo_pool_size,
 		0, 0, INT_MAX,
@@ -2024,7 +2049,8 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"geqo_generations", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("GEQO: number of iterations of the algorithm."),
-			gettext_noop("Zero selects a suitable default value.")
+			gettext_noop("Zero selects a suitable default value."),
+			GUC_EXPLAIN
 		},
 		&Geqo_generations,
 		0, 0, INT_MAX,
@@ -2138,7 +2164,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"temp_buffers", PGC_USERSET, RESOURCES_MEM,
 			gettext_noop("Sets the maximum number of temporary buffers used by each session."),
 			NULL,
-			GUC_UNIT_BLOCKS
+			GUC_UNIT_BLOCKS | GUC_EXPLAIN
 		},
 		&num_temp_buffers,
 		1024, 100, INT_MAX / 2,
@@ -2205,7 +2231,7 @@ static struct config_int ConfigureNamesInt[] =
 			gettext_noop("This much memory can be used by each internal "
 						 "sort operation and hash table before switching to "
 						 "temporary disk files."),
-			GUC_UNIT_KB
+			GUC_UNIT_KB | GUC_EXPLAIN
 		},
 		&work_mem,
 		4096, 64, MAX_KILOBYTES,
@@ -2734,7 +2760,8 @@ static struct config_int ConfigureNamesInt[] =
 			PGC_USERSET,
 			RESOURCES_ASYNCHRONOUS,
 			gettext_noop("Number of simultaneous requests that can be handled efficiently by the disk subsystem."),
-			gettext_noop("For RAID arrays, this should be approximately the number of drive spindles in the array.")
+			gettext_noop("For RAID arrays, this should be approximately the number of drive spindles in the array."),
+			GUC_EXPLAIN
 		},
 		&effective_io_concurrency,
 #ifdef USE_PREFETCH
@@ -2979,7 +3006,8 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"max_parallel_workers_per_gather", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
 			gettext_noop("Sets the maximum number of parallel processes per executor node."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&max_parallel_workers_per_gather,
 		2, 0, MAX_PARALLEL_WORKER_LIMIT,
@@ -2989,7 +3017,8 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"max_parallel_workers", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
 			gettext_noop("Sets the maximum number of parallel workers that can be active at one time."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&max_parallel_workers,
 		8, 0, MAX_PARALLEL_WORKER_LIMIT,
@@ -3079,7 +3108,7 @@ static struct config_int ConfigureNamesInt[] =
 			gettext_noop("Sets the planner's assumption about the total size of the data caches."),
 			gettext_noop("That is, the total size of the caches (kernel cache and shared buffers) used for PostgreSQL data files. "
 						 "This is measured in disk pages, which are normally 8 kB each."),
-			GUC_UNIT_BLOCKS,
+			GUC_UNIT_BLOCKS | GUC_EXPLAIN,
 		},
 		&effective_cache_size,
 		DEFAULT_EFFECTIVE_CACHE_SIZE, 1, INT_MAX,
@@ -3090,7 +3119,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"min_parallel_table_scan_size", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the minimum amount of table data for a parallel scan."),
 			gettext_noop("If the planner estimates that it will read a number of table pages too small to reach this limit, a parallel scan will not be considered."),
-			GUC_UNIT_BLOCKS,
+			GUC_UNIT_BLOCKS | GUC_EXPLAIN,
 		},
 		&min_parallel_table_scan_size,
 		(8 * 1024 * 1024) / BLCKSZ, 0, INT_MAX / 3,
@@ -3101,7 +3130,7 @@ static struct config_int ConfigureNamesInt[] =
 		{"min_parallel_index_scan_size", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the minimum amount of index data for a parallel scan."),
 			gettext_noop("If the planner estimates that it will read a number of index pages too small to reach this limit, a parallel scan will not be considered."),
-			GUC_UNIT_BLOCKS,
+			GUC_UNIT_BLOCKS | GUC_EXPLAIN,
 		},
 		&min_parallel_index_scan_size,
 		(512 * 1024) / BLCKSZ, 0, INT_MAX / 3,
@@ -3166,7 +3195,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"seq_page_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of a "
 						 "sequentially fetched disk page."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&seq_page_cost,
 		DEFAULT_SEQ_PAGE_COST, 0, DBL_MAX,
@@ -3176,7 +3206,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"random_page_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of a "
 						 "nonsequentially fetched disk page."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&random_page_cost,
 		DEFAULT_RANDOM_PAGE_COST, 0, DBL_MAX,
@@ -3186,7 +3217,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"cpu_tuple_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of "
 						 "processing each tuple (row)."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&cpu_tuple_cost,
 		DEFAULT_CPU_TUPLE_COST, 0, DBL_MAX,
@@ -3196,7 +3228,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"cpu_index_tuple_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of "
 						 "processing each index entry during an index scan."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&cpu_index_tuple_cost,
 		DEFAULT_CPU_INDEX_TUPLE_COST, 0, DBL_MAX,
@@ -3206,7 +3239,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"cpu_operator_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of "
 						 "processing each operator or function call."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&cpu_operator_cost,
 		DEFAULT_CPU_OPERATOR_COST, 0, DBL_MAX,
@@ -3216,7 +3250,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"parallel_tuple_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of "
 						 "passing each tuple (row) from worker to master backend."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&parallel_tuple_cost,
 		DEFAULT_PARALLEL_TUPLE_COST, 0, DBL_MAX,
@@ -3226,7 +3261,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"parallel_setup_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Sets the planner's estimate of the cost of "
 						 "starting up worker processes for parallel query."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&parallel_setup_cost,
 		DEFAULT_PARALLEL_SETUP_COST, 0, DBL_MAX,
@@ -3236,7 +3272,8 @@ static struct config_real ConfigureNamesReal[] =
 	{
 		{"jit_above_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Perform JIT compilation if query is more expensive."),
-			gettext_noop("-1 disables JIT compilation.")
+			gettext_noop("-1 disables JIT compilation."),
+			GUC_EXPLAIN
 		},
 		&jit_above_cost,
 		100000, -1, DBL_MAX,
@@ -3246,7 +3283,8 @@ static struct config_real ConfigureNamesReal[] =
 	{
 		{"jit_optimize_above_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Optimize JITed functions if query is more expensive."),
-			gettext_noop("-1 disables optimization.")
+			gettext_noop("-1 disables optimization."),
+			GUC_EXPLAIN
 		},
 		&jit_optimize_above_cost,
 		500000, -1, DBL_MAX,
@@ -3256,7 +3294,8 @@ static struct config_real ConfigureNamesReal[] =
 	{
 		{"jit_inline_above_cost", PGC_USERSET, QUERY_TUNING_COST,
 			gettext_noop("Perform JIT inlining if query is more expensive."),
-			gettext_noop("-1 disables inlining.")
+			gettext_noop("-1 disables inlining."),
+			GUC_EXPLAIN
 		},
 		&jit_inline_above_cost,
 		500000, -1, DBL_MAX,
@@ -3267,7 +3306,8 @@ static struct config_real ConfigureNamesReal[] =
 		{"cursor_tuple_fraction", PGC_USERSET, QUERY_TUNING_OTHER,
 			gettext_noop("Sets the planner's estimate of the fraction of "
 						 "a cursor's rows that will be retrieved."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&cursor_tuple_fraction,
 		DEFAULT_CURSOR_TUPLE_FRACTION, 0.0, 1.0,
@@ -3277,7 +3317,8 @@ static struct config_real ConfigureNamesReal[] =
 	{
 		{"geqo_selection_bias", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("GEQO: selective pressure within the population."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&Geqo_selection_bias,
 		DEFAULT_GEQO_SELECTION_BIAS,
@@ -3287,7 +3328,8 @@ static struct config_real ConfigureNamesReal[] =
 	{
 		{"geqo_seed", PGC_USERSET, QUERY_TUNING_GEQO,
 			gettext_noop("GEQO: seed for random path selection."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&Geqo_seed,
 		0.0, 0.0, 1.0,
@@ -3747,7 +3789,7 @@ static struct config_string ConfigureNamesString[] =
 		{"search_path", PGC_USERSET, CLIENT_CONN_STATEMENT,
 			gettext_noop("Sets the schema search order for names that are not schema-qualified."),
 			NULL,
-			GUC_LIST_INPUT | GUC_LIST_QUOTE
+			GUC_LIST_INPUT | GUC_LIST_QUOTE | GUC_EXPLAIN
 		},
 		&namespace_search_path,
 		"\"$user\", public",
@@ -4200,7 +4242,8 @@ static struct config_enum ConfigureNamesEnum[] =
 		{"constraint_exclusion", PGC_USERSET, QUERY_TUNING_OTHER,
 			gettext_noop("Enables the planner to use constraints to optimize queries."),
 			gettext_noop("Table scans will be skipped if their constraints"
-						 " guarantee that no rows match the query.")
+						 " guarantee that no rows match the query."),
+			GUC_EXPLAIN
 		},
 		&constraint_exclusion,
 		CONSTRAINT_EXCLUSION_PARTITION, constraint_exclusion_options,
@@ -4436,7 +4479,8 @@ static struct config_enum ConfigureNamesEnum[] =
 	{
 		{"force_parallel_mode", PGC_USERSET, QUERY_TUNING_OTHER,
 			gettext_noop("Forces use of parallel query facilities."),
-			gettext_noop("If possible, run query using a parallel worker and with parallel restrictions.")
+			gettext_noop("If possible, run query using a parallel worker and with parallel restrictions."),
+			GUC_EXPLAIN
 		},
 		&force_parallel_mode,
 		FORCE_PARALLEL_OFF, force_parallel_mode_options,
@@ -4460,7 +4504,8 @@ static struct config_enum ConfigureNamesEnum[] =
 			gettext_noop("Controls the planner's selection of custom or generic plan."),
 			gettext_noop("Prepared statements can have custom and generic plans, and the planner "
 						 "will attempt to choose which is better.  This can be set to override "
-						 "the default behavior.")
+						 "the default behavior."),
+			GUC_EXPLAIN
 		},
 		&plan_cache_mode,
 		PLAN_CACHE_MODE_AUTO, plan_cache_mode_options,
@@ -4521,6 +4566,9 @@ static struct config_generic **guc_variables;
 /* Current number of variables contained in the vector */
 static int	num_guc_variables;
 
+/* Current number of variables marked with GUC_EXPLAIN */
+static int	num_guc_explain_variables;
+
 /* Vector capacity */
 static int	size_guc_variables;
 
@@ -4784,6 +4832,7 @@ build_guc_variables(void)
 {
 	int			size_vars;
 	int			num_vars = 0;
+	int			num_explain_vars = 0;
 	struct config_generic **guc_vars;
 	int			i;
 
@@ -4794,6 +4843,9 @@ build_guc_variables(void)
 		/* Rather than requiring vartype to be filled in by hand, do this: */
 		conf->gen.vartype = PGC_BOOL;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesInt[i].gen.name; i++)
@@ -4802,6 +4854,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_INT;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesReal[i].gen.name; i++)
@@ -4810,6 +4865,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_REAL;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesString[i].gen.name; i++)
@@ -4818,6 +4876,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_STRING;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesEnum[i].gen.name; i++)
@@ -4826,6 +4887,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_ENUM;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	/*
@@ -4857,6 +4921,7 @@ build_guc_variables(void)
 		free(guc_variables);
 	guc_variables = guc_vars;
 	num_guc_variables = num_vars;
+	num_guc_explain_variables = num_explain_vars;
 	size_guc_variables = size_vars;
 	qsort((void *) guc_variables, num_guc_variables,
 		  sizeof(struct config_generic *), guc_var_compare);
@@ -8807,6 +8872,99 @@ ShowAllGUCConfig(DestReceiver *dest)
 	end_tup_output(tstate);
 }
 
+/*
+ * Returns an array of modified GUC options to show in EXPLAIN. Only options
+ * related to query planning (marked with GUC_EXPLAIN), with values different
+ * from built-in defaults.
+ */
+struct config_generic **
+get_explain_guc_options(int *num)
+{
+	int		i;
+	struct config_generic **result;
+
+	*num = 0;
+
+	/*
+	 * Allocate enough space to fit all GUC_EXPLAIN options. We may not
+	 * need all the space, but there are fairly few such options so we
+	 * don't waste a lot of memory.
+	 */
+	result = palloc(sizeof(struct config_generic *) * num_guc_explain_variables);
+
+	for (i = 0; i < num_guc_variables; i++)
+	{
+		bool modified;
+		struct config_generic *conf = guc_variables[i];
+
+		/* return only options visible to the user */
+		if ((conf->flags & GUC_NO_SHOW_ALL) ||
+			((conf->flags & GUC_SUPERUSER_ONLY) &&
+			 !is_member_of_role(GetUserId(), DEFAULT_ROLE_READ_ALL_SETTINGS)))
+			continue;
+
+		/* only parameters explicitly marked for inclusion in explain */
+		if (!(conf->flags & GUC_EXPLAIN))
+			continue;
+
+		/* return only options that were modified (w.r.t. config file) */
+		modified = false;
+
+		switch (conf->vartype)
+		{
+			case PGC_BOOL:
+			{
+				struct config_bool *lconf = (struct config_bool *) conf;
+				modified = (lconf->boot_val != *(lconf->variable));
+			}
+			break;
+
+			case PGC_INT:
+				{
+					struct config_int *lconf = (struct config_int *) conf;
+					modified = (lconf->boot_val != *(lconf->variable));
+				}
+				break;
+
+			case PGC_REAL:
+				{
+					struct config_real *lconf = (struct config_real *) conf;
+					modified = (lconf->boot_val != *(lconf->variable));
+				}
+				break;
+
+			case PGC_STRING:
+				{
+					struct config_string *lconf = (struct config_string *) conf;
+					modified = (strcmp(lconf->boot_val, *(lconf->variable)) != 0);
+				}
+				break;
+
+			case PGC_ENUM:
+				{
+					struct config_enum *lconf = (struct config_enum *) conf;
+					modified = (lconf->boot_val != *(lconf->variable));
+				}
+				break;
+
+			default:
+				elog(ERROR, "unexcpected GUC type: %d", conf->vartype);
+		}
+
+		/* skip GUC variables that match the built-in default */
+		if (!modified)
+			continue;
+
+		/* assign to the values array */
+		result[*num] = conf;
+		*num = *num + 1;
+
+		Assert(*num <= num_guc_explain_variables);
+	}
+
+	return result;
+}
+
 /*
  * Return GUC variable value by name; optionally return canonical form of
  * name.  If the GUC is unset, then throw an error unless missing_ok is true,

src/include/commands/explain.h

@@ -35,6 +35,7 @@ typedef struct ExplainState
 	bool		buffers;		/* print buffer usage */
 	bool		timing;			/* print detailed node timing */
 	bool		summary;		/* print total planning and execution timing */
+	bool		settings;		/* print modified settings */
 	ExplainFormat format;		/* output format */
 	/* state for output formatting --- not reset for each new plan tree */
 	int			indent;			/* current indentation level */

src/include/utils/guc.h

@@ -227,6 +227,8 @@ typedef enum
 #define GUC_UNIT_MIN		   0x30000	/* value is in minutes */
 #define GUC_UNIT_TIME		   0xF0000	/* mask for time-related units */
 
+#define GUC_EXPLAIN			  0x100000	/* include in explain */
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 

src/include/utils/guc_tables.h

@@ -267,5 +267,6 @@ extern void build_guc_variables(void);
 extern const char *config_enum_lookup_by_value(struct config_enum *record, int val);
 extern bool config_enum_lookup_by_name(struct config_enum *record,
 						   const char *value, int *retval);
+extern struct config_generic **get_explain_guc_options(int *num);
 
 #endif							/* GUC_TABLES_H */