Attached is v4, changing how GUCs are picked for inclusion on the query
plans. Instead of picking the GUCs based on group and/or explicitly, a
new GUC_EXPLAIN flag is used for that.

I went through GUCs defined in guc.c and marked those in QUERY_TUNING*
groups accordingly, with the exception of default_statistics_target
because that seems somewhat useless without showing the value used to
actually analyze the table (and/or columns).

I've also included a couple of other GUCs, that I find to be relevant:

- parallel_leader_participation
- max_parallel_workers_per_gather
- max_parallel_workers
- search_path
- effective_io_concurrency
- work_mem
- temp_buffers
- plan_cache_mode

I think this covers the interesting GUCs pretty well, although perhaps I
missed something.

The one bit that needs fixing is escaping the GUC values when showing
them in the plan. Looking at the other places that currently escape
stuff, I see they only care about YAML/JSON/XML and leave the regular
output unescaped. I was wondering if it's OK with the current format
with all GUCs on a single line

                    QUERY PLAN
---------------------------------------------------
 Seq Scan on t  (cost=0.00..54.63 rows=13 width=4)
   Filter: ('x''y'::text = (a)::text)
 GUCs: enable_nestloop = 'off', work_mem = '32MB'
(3 rows)

but I suppose it is, because without the escaping a user can break
whatever format we use. So I'll do the same thing, escaping just the
structured formats (YAML et al).

The question however is whether someone has a better formatting idea?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

diff --git a/contrib/auto_explain/auto_explain.c b/contrib/auto_explain/auto_explain.c
index 646cd0d42c..ec44c59b7f 100644
--- a/contrib/auto_explain/auto_explain.c
+++ b/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_gucs = 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_gucs",
+							 "Print modified GUC values.",
+							 NULL,
+							 &auto_explain_log_gucs,
+							 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->gucs = auto_explain_log_gucs;
 
 			ExplainBeginOutput(es);
 			ExplainQueryText(es, queryDesc);
diff --git a/doc/src/sgml/auto-explain.sgml b/doc/src/sgml/auto-explain.sgml
index 120b168d45..852c69b7bb 100644
--- a/doc/src/sgml/auto-explain.sgml
+++ b/doc/src/sgml/auto-explain.sgml
@@ -169,6 +169,23 @@ LOAD 'auto_explain';
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term>
+     <varname>auto_explain.log_gucs</varname> (<type>boolean</type>)
+     <indexterm>
+      <primary><varname>auto_explain.log_gucs</varname> configuration parameter</primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      <varname>auto_explain.log_gucs</varname> controls whether information
+      about modified configuration options are logged with the execution
+      plan.  Only options modified at the database, user, client or session
+      level are considered modified.  This parameter is off by default.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term>
      <varname>auto_explain.log_format</varname> (<type>enum</type>)
diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index 094e977fb5..e32497d30d 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -30,6 +30,7 @@
 #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"
@@ -163,6 +164,8 @@ 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, "gucs") == 0)
+			es->gucs = defGetBoolean(opt);
 		else if (strcmp(opt->defname, "timing") == 0)
 		{
 			timing_set = true;
@@ -634,6 +637,41 @@ 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 that don't
+	 * match the built-in defaults.
+	 */
+	if (es->gucs)
+	{
+		int		i;
+		int		num;
+		StringInfoData	str;
+		struct config_generic **gucs;
+
+		gucs = get_explain_guc_options(&num);
+
+		for (i = 0; i < num; i++)
+		{
+			char *setting;
+			struct config_generic *conf = gucs[i];
+
+			if (i == 0)
+				initStringInfo(&str);
+			else
+				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("GUCs", str.data, es);
+	}
 }
 
 /*
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 6fe1939881..c2f1bb956d 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -862,7 +862,8 @@ 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,
@@ -871,7 +872,8 @@ 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,
@@ -880,7 +882,8 @@ 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,
@@ -889,7 +892,8 @@ 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,
@@ -898,7 +902,8 @@ 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,
@@ -907,7 +912,8 @@ 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,
@@ -916,7 +922,8 @@ 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,
@@ -925,7 +932,8 @@ 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,
@@ -934,7 +942,8 @@ 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,
@@ -943,7 +952,8 @@ 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,
@@ -952,7 +962,8 @@ 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,
@@ -961,7 +972,8 @@ 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,
@@ -970,7 +982,8 @@ 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,
@@ -979,7 +992,8 @@ 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,
@@ -988,7 +1002,8 @@ 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,
@@ -997,7 +1012,8 @@ 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,
@@ -1008,7 +1024,8 @@ 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,
@@ -1018,7 +1035,8 @@ 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,
@@ -1591,7 +1609,7 @@ 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,
@@ -1782,7 +1800,8 @@ 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,
@@ -1792,7 +1811,8 @@ static struct config_bool ConfigureNamesBool[] =
 	{
 		{"jit", PGC_USERSET, QUERY_TUNING_OTHER,
 			gettext_noop("Allow JIT compilation."),
-			NULL
+			NULL,
+			GUC_EXPLAIN
 		},
 		&jit_enabled,
 		true,
@@ -1921,7 +1941,8 @@ 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,
@@ -1933,7 +1954,8 @@ 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,
@@ -1942,7 +1964,8 @@ 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,
@@ -1951,7 +1974,8 @@ 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,
@@ -1960,7 +1984,8 @@ 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,
@@ -1969,7 +1994,8 @@ 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,
@@ -2083,7 +2109,7 @@ 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,
@@ -2150,7 +2176,7 @@ 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,
@@ -2701,7 +2727,8 @@ 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
@@ -2946,7 +2973,8 @@ 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,
@@ -2956,7 +2984,8 @@ 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,
@@ -3046,7 +3075,7 @@ 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,
@@ -3057,7 +3086,7 @@ 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,
@@ -3068,7 +3097,7 @@ 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,
@@ -3133,7 +3162,8 @@ 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,
@@ -3143,7 +3173,8 @@ 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,
@@ -3153,7 +3184,8 @@ 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,
@@ -3163,7 +3195,8 @@ 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,
@@ -3173,7 +3206,8 @@ 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,
@@ -3183,7 +3217,8 @@ 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,
@@ -3193,7 +3228,8 @@ 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,
@@ -3203,7 +3239,8 @@ 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,
@@ -3213,7 +3250,8 @@ 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,
@@ -3223,7 +3261,8 @@ 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,
@@ -3234,7 +3273,8 @@ 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,
@@ -3244,7 +3284,8 @@ 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,
@@ -3254,7 +3295,8 @@ 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,
@@ -3669,7 +3711,7 @@ 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",
@@ -4122,7 +4164,8 @@ 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,
@@ -4348,7 +4391,8 @@ 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,
@@ -4372,7 +4416,8 @@ 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,
@@ -8556,6 +8601,86 @@ ShowAllGUCConfig(DestReceiver *dest)
 	end_tup_output(tstate);
 }
 
+struct config_generic **
+get_explain_guc_options(int *num)
+{
+	int		i;
+	struct config_generic **result;
+
+	*num = 0;
+	result = palloc(sizeof(struct config_generic *) * num_guc_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;
+	}
+
+	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,
diff --git a/src/include/commands/explain.h b/src/include/commands/explain.h
index d3f70fda08..05afca22aa 100644
--- a/src/include/commands/explain.h
+++ b/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		gucs;			/* print modified GUCs */
 	ExplainFormat format;		/* output format */
 	/* state for output formatting --- not reset for each new plan tree */
 	int			indent;			/* current indentation level */
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index 64457c792a..f316320efc 100644
--- a/src/include/utils/guc.h
+++ b/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)
 
 
diff --git a/src/include/utils/guc_tables.h b/src/include/utils/guc_tables.h
index 6f9fdb6a5f..641cb29fbe 100644
--- a/src/include/utils/guc_tables.h
+++ b/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 */

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