Disallow creation of indexes on system columns (except for OID).

Although OID acts pretty much like user data, the other system columns do
not, so an index on one would likely misbehave.  And it's pretty hard to
see a use-case for one, anyway.  Let's just forbid the case rather than
worry about whether it should be supported.

David Rowley

src/test/regress/sql/create_index.sql

@@ -824,6 +824,25 @@ DROP INDEX cwi_replaced_pkey;	-- Should fail; a constraint depends on it
 
 DROP TABLE cwi_test;
 
+--
+-- Check handling of indexes on system columns
+--
+CREATE TABLE oid_table (a INT) WITH OIDS;
+
+-- An index on the OID column should be allowed
+CREATE INDEX ON oid_table (oid);
+
+-- Other system columns cannot be indexed
+CREATE INDEX ON oid_table (ctid);
+
+-- nor used in expressions
+CREATE INDEX ON oid_table ((ctid >= '(1000,0)'));
+
+-- nor used in predicates
+CREATE INDEX ON oid_table (a) WHERE ctid >= '(1000,0)';
+
+DROP TABLE oid_table;
+
 --
 -- Tests for IS NULL/IS NOT NULL with b-tree indexes
 --