Fix EXPLAIN (SETTINGS) to follow policy about when to print empty fields.

In non-TEXT output formats, the "Settings" field should appear when
requested, even if it would be empty.

Also, get rid of the premature optimization of counting all the
GUC_EXPLAIN variables at startup.  Since there was no provision for
adjusting that count later, all it'd take would be some extension marking
a parameter as GUC_EXPLAIN to risk an assertion failure or memory stomp.
We could make get_explain_guc_options() count those variables on-the-fly,
or dynamically resize its array ... but TBH I do not think that making a
transient array of pointers a bit smaller is worth any extra complication,
especially when you consider all the other transient space EXPLAIN eats.
So just allocate that array at the max possible size.

In HEAD, also add some regression test coverage for this feature.

Because of the memory-stomp hazard, back-patch to v12 where this
feature was added.

Discussion: https://postgr.es/m/19416.1580069629@sss.pgh.pa.us

src/backend/commands/explain.c

@@ -626,17 +626,11 @@ ExplainPrintSettings(ExplainState *es)
 	/* 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++)
+		for (int i = 0; i < num; i++)
 		{
 			char	   *setting;
 			struct config_generic *conf = gucs[i];
@@ -650,12 +644,15 @@ ExplainPrintSettings(ExplainState *es)
 	}
 	else
 	{
-		int			i;
 		StringInfoData str;
 
+		/* In TEXT mode, print nothing if there are no options */
+		if (num <= 0)
+			return;
+
 		initStringInfo(&str);
 
-		for (i = 0; i < num; i++)
+		for (int i = 0; i < num; i++)
 		{
 			char	   *setting;
 			struct config_generic *conf = gucs[i];
@@ -671,8 +668,7 @@ ExplainPrintSettings(ExplainState *es)
 				appendStringInfo(&str, "%s = NULL", conf->name);
 		}
 
-		if (num > 0)
-			ExplainPropertyText("Settings", str.data, es);
+		ExplainPropertyText("Settings", str.data, es);
 	}
 }
 

src/backend/utils/misc/guc.c

@@ -4663,9 +4663,6 @@ 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;
 
@@ -4929,7 +4926,6 @@ build_guc_variables(void)
 {
 	int			size_vars;
 	int			num_vars = 0;
-	int			num_explain_vars = 0;
 	struct config_generic **guc_vars;
 	int			i;
 
@@ -4940,9 +4936,6 @@ 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++)
@@ -4951,9 +4944,6 @@ 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++)
@@ -4962,9 +4952,6 @@ 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++)
@@ -4973,9 +4960,6 @@ 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++)
@@ -4984,9 +4968,6 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_ENUM;
 		num_vars++;
-
-		if (conf->gen.flags & GUC_EXPLAIN)
-			num_explain_vars++;
 	}
 
 	/*
@@ -5018,7 +4999,6 @@ 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);
@@ -8967,41 +8947,40 @@ ShowAllGUCConfig(DestReceiver *dest)
 }
 
 /*
- * 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.
+ * Return an array of modified GUC options to show in EXPLAIN.
+ *
+ * We only report options related to query planning (marked with GUC_EXPLAIN),
+ * with values different from their 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.
+	 * While only a fraction of all the GUC variables are marked GUC_EXPLAIN,
+	 * it doesn't seem worth dynamically resizing this array.
 	 */
-	result = palloc(sizeof(struct config_generic *) * num_guc_explain_variables);
+	result = palloc(sizeof(struct config_generic *) * num_guc_variables);
 
-	for (i = 0; i < num_guc_variables; i++)
+	for (int i = 0; i < num_guc_variables; i++)
 	{
 		bool		modified;
 		struct config_generic *conf = guc_variables[i];
 
-		/* return only options visible to the user */
+		/* return only parameters marked for inclusion in explain */
+		if (!(conf->flags & GUC_EXPLAIN))
+			continue;
+
+		/* return only options visible to the current 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) */
+		/* return only options that are different from their boot values */
 		modified = false;
 
 		switch (conf->vartype)
@@ -9050,15 +9029,12 @@ get_explain_guc_options(int *num)
 				elog(ERROR, "unexpected GUC type: %d", conf->vartype);
 		}
 
-		/* skip GUC variables that match the built-in default */
 		if (!modified)
 			continue;
 
-		/* assign to the values array */
+		/* OK, report it */
 		result[*num] = conf;
 		*num = *num + 1;
-
-		Assert(*num <= num_guc_explain_variables);
 	}
 
 	return result;

src/test/regress/expected/explain.out

@@ -181,6 +181,26 @@ select explain_filter('explain (analyze, buffers, format yaml) select * from int
    Execution Time: N.N
 (1 row)
 
+-- SETTINGS option
+-- We have to ignore other settings that might be imposed by the environment,
+-- so printing the whole Settings field unfortunately won't do.
+begin;
+set local plan_cache_mode = force_generic_plan;
+select true as "OK"
+  from explain_filter('explain (settings) select * from int8_tbl i8') ln
+  where ln ~ '^ *Settings: .*plan_cache_mode = ''force_generic_plan''';
+ OK 
+----
+ t
+(1 row)
+
+select explain_filter_to_json('explain (settings, format json) select * from int8_tbl i8') #> '{0,Settings,plan_cache_mode}';
+       ?column?       
+----------------------
+ "force_generic_plan"
+(1 row)
+
+rollback;
 --
 -- Test production of per-worker data
 --

src/test/regress/sql/explain.sql

@@ -57,6 +57,18 @@ select explain_filter('explain (analyze, buffers, format json) select * from int
 select explain_filter('explain (analyze, buffers, format xml) select * from int8_tbl i8');
 select explain_filter('explain (analyze, buffers, format yaml) select * from int8_tbl i8');
 
+-- SETTINGS option
+-- We have to ignore other settings that might be imposed by the environment,
+-- so printing the whole Settings field unfortunately won't do.
+
+begin;
+set local plan_cache_mode = force_generic_plan;
+select true as "OK"
+  from explain_filter('explain (settings) select * from int8_tbl i8') ln
+  where ln ~ '^ *Settings: .*plan_cache_mode = ''force_generic_plan''';
+select explain_filter_to_json('explain (settings, format json) select * from int8_tbl i8') #> '{0,Settings,plan_cache_mode}';
+rollback;
+
 --
 -- Test production of per-worker data
 --