Disallow CREATE STATISTICS on system catalogs

Add a check that CREATE STATISTICS does not add extended statistics on
system catalogs, similarly to indexes etc.  It can be overriden using
the allow_system_table_mods GUC.

This bug exists since 7b504eb282, adding the extended statistics, so
backpatch all the way back to PostgreSQL 10.

Author: Tomas Vondra
Reported-by: Dean Rasheed
Backpatch-through: 10
Discussion: https://postgr.es/m/CAEZATCXAPrrOKwEsyZKQ4uzzJQWBCt6QAvOcgqRGdWwT1zb%2BrQ%40mail.gmail.com

src/backend/commands/statscmds.c

@@ -135,6 +135,13 @@ CreateStatistics(CreateStatsStmt *stmt)
 		if (!pg_class_ownercheck(RelationGetRelid(rel), stxowner))
 			aclcheck_error(ACLCHECK_NOT_OWNER, get_relkind_objtype(rel->rd_rel->relkind),
 						   RelationGetRelationName(rel));
+
+		/* Creating statistics on system catalogs is not allowed */
+		if (!allowSystemTableMods && IsSystemRelation(rel))
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission denied: \"%s\" is a system catalog",
+							RelationGetRelationName(rel))));
 	}
 
 	Assert(rel);

src/test/regress/expected/stats_ext.out

@@ -25,6 +25,7 @@ begin
 end;
 $$;
 -- Verify failures
+CREATE TABLE ext_stats_test (x int, y int, z int);
 CREATE STATISTICS tst;
 ERROR:  syntax error at or near ";"
 LINE 1: CREATE STATISTICS tst;
@@ -39,16 +40,17 @@ LINE 1: CREATE STATISTICS tst FROM sometab;
                               ^
 CREATE STATISTICS tst ON a, b FROM nonexistent;
 ERROR:  relation "nonexistent" does not exist
-CREATE STATISTICS tst ON a, b FROM pg_class;
+CREATE STATISTICS tst ON a, b FROM ext_stats_test;
 ERROR:  column "a" does not exist
-CREATE STATISTICS tst ON relname, relname, relnatts FROM pg_class;
+CREATE STATISTICS tst ON x, x, y FROM ext_stats_test;
 ERROR:  duplicate column name in statistics definition
-CREATE STATISTICS tst ON relnatts + relpages FROM pg_class;
+CREATE STATISTICS tst ON x + y FROM ext_stats_test;
 ERROR:  only simple column references are allowed in CREATE STATISTICS
-CREATE STATISTICS tst ON (relpages, reltuples) FROM pg_class;
+CREATE STATISTICS tst ON (x, y) FROM ext_stats_test;
 ERROR:  only simple column references are allowed in CREATE STATISTICS
-CREATE STATISTICS tst (unrecognized) ON relname, relnatts FROM pg_class;
+CREATE STATISTICS tst (unrecognized) ON x, y FROM ext_stats_test;
 ERROR:  unrecognized statistics kind "unrecognized"
+DROP TABLE ext_stats_test;
 -- Ensure stats are dropped sanely, and test IF NOT EXISTS while at it
 CREATE TABLE ab1 (a INTEGER, b INTEGER, c INTEGER);
 CREATE STATISTICS IF NOT EXISTS ab1_a_b_stats ON a, b FROM ab1;

src/test/regress/sql/stats_ext.sql

@@ -28,15 +28,17 @@ end;
 $$;
 
 -- Verify failures
+CREATE TABLE ext_stats_test (x int, y int, z int);
 CREATE STATISTICS tst;
 CREATE STATISTICS tst ON a, b;
 CREATE STATISTICS tst FROM sometab;
 CREATE STATISTICS tst ON a, b FROM nonexistent;
-CREATE STATISTICS tst ON a, b FROM pg_class;
-CREATE STATISTICS tst ON relname, relname, relnatts FROM pg_class;
-CREATE STATISTICS tst ON relnatts + relpages FROM pg_class;
-CREATE STATISTICS tst ON (relpages, reltuples) FROM pg_class;
-CREATE STATISTICS tst (unrecognized) ON relname, relnatts FROM pg_class;
+CREATE STATISTICS tst ON a, b FROM ext_stats_test;
+CREATE STATISTICS tst ON x, x, y FROM ext_stats_test;
+CREATE STATISTICS tst ON x + y FROM ext_stats_test;
+CREATE STATISTICS tst ON (x, y) FROM ext_stats_test;
+CREATE STATISTICS tst (unrecognized) ON x, y FROM ext_stats_test;
+DROP TABLE ext_stats_test;
 
 -- Ensure stats are dropped sanely, and test IF NOT EXISTS while at it
 CREATE TABLE ab1 (a INTEGER, b INTEGER, c INTEGER);