diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 978b385568..93da5d24ba 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1383,11 +1383,14 @@ check_GUC_name_for_parameter_acl(const char *name)
 }
 
 /*
- * Routine in charge of checking that the initial value of a GUC is the
- * same when declared and when loaded to prevent anybody looking at the
- * C declarations of these GUCS from being fooled by mismatched values.
+ * Routine in charge of checking various states of a GUC.
  *
- * The following validation rules apply:
+ * This performs two sanity checks.  First, it checks that the initial
+ * value of a GUC is the same when declared and when loaded to prevent
+ * anybody looking at the C declarations of these GUCS from being fooled by
+ * mismatched values.  Second, it checks for incorrect flag combinations.
+ *
+ * The following validation rules apply for the values:
  * bool - can be false, otherwise must be same as the boot_val
  * int  - can be 0, otherwise must be same as the boot_val
  * real - can be 0.0, otherwise must be same as the boot_val
@@ -1398,6 +1401,7 @@ check_GUC_name_for_parameter_acl(const char *name)
 static bool
 check_GUC_init(struct config_generic *gconf)
 {
+	/* Checks on values */
 	switch (gconf->vartype)
 	{
 		case PGC_BOOL:
@@ -1462,6 +1466,16 @@ check_GUC_init(struct config_generic *gconf)
 			}
 	}
 
+	/* Flag combinations */
+	/* GUC_NO_SHOW_ALL requires GUC_NOT_IN_SAMPLE */
+	if ((gconf->flags & GUC_NO_SHOW_ALL) &&
+		!(gconf->flags & GUC_NOT_IN_SAMPLE))
+	{
+		elog(LOG, "GUC %s flags: NO_SHOW_ALL and !NOT_IN_SAMPLE",
+			 gconf->name);
+		return false;
+	}
+
 	return true;
 }
 #endif
diff --git a/src/test/regress/expected/guc.out b/src/test/regress/expected/guc.out
index 2914b950b4..3185ec0063 100644
--- a/src/test/regress/expected/guc.out
+++ b/src/test/regress/expected/guc.out
@@ -880,15 +880,7 @@ SELECT name FROM tab_settings_flags
 ------
 (0 rows)
 
--- NO_SHOW_ALL implies NO_RESET_ALL, and vice-versa.
-SELECT name FROM tab_settings_flags
-  WHERE no_show_all AND NOT no_reset_all
-  ORDER BY 1;
- name 
-------
-(0 rows)
-
--- Exceptions are transaction_*.
+-- NO_RESET_ALL can be specified without NO_SHOW_ALL, like transaction_*.
 SELECT name FROM tab_settings_flags
   WHERE NOT no_show_all AND no_reset_all
   ORDER BY 1;
@@ -899,14 +891,6 @@ SELECT name FROM tab_settings_flags
  transaction_read_only
 (3 rows)
 
--- NO_SHOW_ALL implies NOT_IN_SAMPLE.
-SELECT name FROM tab_settings_flags
-  WHERE no_show_all AND NOT not_in_sample
-  ORDER BY 1;
- name 
-------
-(0 rows)
-
 -- NO_RESET implies NO_RESET_ALL.
 SELECT name FROM tab_settings_flags
   WHERE no_reset AND NOT no_reset_all
diff --git a/src/test/regress/sql/guc.sql b/src/test/regress/sql/guc.sql
index d40d0c178f..f672b88b54 100644
--- a/src/test/regress/sql/guc.sql
+++ b/src/test/regress/sql/guc.sql
@@ -349,18 +349,10 @@ SELECT name FROM tab_settings_flags
 SELECT name FROM tab_settings_flags
   WHERE category = 'Preset Options' AND NOT not_in_sample
   ORDER BY 1;
--- NO_SHOW_ALL implies NO_RESET_ALL, and vice-versa.
-SELECT name FROM tab_settings_flags
-  WHERE no_show_all AND NOT no_reset_all
-  ORDER BY 1;
--- Exceptions are transaction_*.
+-- NO_RESET_ALL can be specified without NO_SHOW_ALL, like transaction_*.
 SELECT name FROM tab_settings_flags
   WHERE NOT no_show_all AND no_reset_all
   ORDER BY 1;
--- NO_SHOW_ALL implies NOT_IN_SAMPLE.
-SELECT name FROM tab_settings_flags
-  WHERE no_show_all AND NOT not_in_sample
-  ORDER BY 1;
 -- NO_RESET implies NO_RESET_ALL.
 SELECT name FROM tab_settings_flags
   WHERE no_reset AND NOT no_reset_all