Disallow modifying statistics on system columns.

Reported-by: Heikki Linnakangas
Discussion: https://postgr.es/m/df3e1c41-4e6c-40ad-9636-98deefe488cd@iki.fi

src/backend/statistics/attribute_stats.c

@@ -167,6 +167,13 @@ attribute_statistics_update(FunctionCallInfo fcinfo, int elevel)
 	stats_check_required_arg(fcinfo, attarginfo, ATTNAME_ARG);
 	attname = PG_GETARG_NAME(ATTNAME_ARG);
 	attnum = get_attnum(reloid, NameStr(*attname));
+
+	if (attnum < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot modify statistics on system column \"%s\"",
+						NameStr(*attname))));
+
 	if (attnum == InvalidAttrNumber)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_COLUMN),
@@ -882,6 +889,13 @@ pg_clear_attribute_stats(PG_FUNCTION_ARGS)
 	stats_check_required_arg(fcinfo, attarginfo, ATTNAME_ARG);
 	attname = PG_GETARG_NAME(ATTNAME_ARG);
 	attnum = get_attnum(reloid, NameStr(*attname));
+
+	if (attnum < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot clear statistics on system column \"%s\"",
+						NameStr(*attname))));
+
 	if (attnum == InvalidAttrNumber)
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_COLUMN),

src/test/regress/expected/stats_import.out

@@ -195,6 +195,15 @@ SELECT pg_catalog.pg_set_attribute_stats(
     avg_width => 2::integer,
     n_distinct => 0.3::real);
 ERROR:  "relation" cannot be NULL
+-- error: attribute is system column
+SELECT pg_catalog.pg_set_attribute_stats(
+    relation => 'stats_import.test'::regclass,
+    attname => 'xmin'::name,
+    inherited => false::boolean,
+    null_frac => 0.1::real,
+    avg_width => 2::integer,
+    n_distinct => 0.3::real);
+ERROR:  cannot modify statistics on system column "xmin"
 -- error: attname doesn't exist
 SELECT pg_catalog.pg_set_attribute_stats(
     relation => 'stats_import.test'::regclass,
@@ -204,6 +213,12 @@ SELECT pg_catalog.pg_set_attribute_stats(
     avg_width => 2::integer,
     n_distinct => 0.3::real);
 ERROR:  column "nope" of relation "test" does not exist
+-- error: attribute is system column
+SELECT pg_catalog.pg_clear_attribute_stats(
+    relation => 'stats_import.test'::regclass,
+    attname => 'ctid'::name,
+    inherited => false::boolean);
+ERROR:  cannot clear statistics on system column "ctid"
 -- error: attname doesn't exist
 SELECT pg_catalog.pg_clear_attribute_stats(
     relation => 'stats_import.test'::regclass,

src/test/regress/sql/stats_import.sql

@@ -145,6 +145,15 @@ SELECT pg_catalog.pg_set_attribute_stats(
     avg_width => 2::integer,
     n_distinct => 0.3::real);
 
+-- error: attribute is system column
+SELECT pg_catalog.pg_set_attribute_stats(
+    relation => 'stats_import.test'::regclass,
+    attname => 'xmin'::name,
+    inherited => false::boolean,
+    null_frac => 0.1::real,
+    avg_width => 2::integer,
+    n_distinct => 0.3::real);
+
 -- error: attname doesn't exist
 SELECT pg_catalog.pg_set_attribute_stats(
     relation => 'stats_import.test'::regclass,
@@ -154,6 +163,12 @@ SELECT pg_catalog.pg_set_attribute_stats(
     avg_width => 2::integer,
     n_distinct => 0.3::real);
 
+-- error: attribute is system column
+SELECT pg_catalog.pg_clear_attribute_stats(
+    relation => 'stats_import.test'::regclass,
+    attname => 'ctid'::name,
+    inherited => false::boolean);
+
 -- error: attname doesn't exist
 SELECT pg_catalog.pg_clear_attribute_stats(
     relation => 'stats_import.test'::regclass,