sql/parser: unreserve INDEX and NOTHING from the RHS of SET statements

The SET statement in the pg dialect is special because it
auto-converts identifiers on its RHS to symbolic values or strings. In
particular it is meant to support a diversity of special keywords as
pseudo-values.

This patch ensures that INDEX and NOTHING are accepted on the RHS.

Release note (sql change): the names "index" and "nothing" are again
accepted in the right-hand-side of the assignment in SET statements,
for compatibility with PostgreSQL.

docs/generated/sql/bnf/stmt_block.bnf

@@ -1052,7 +1052,7 @@ to_or_eq ::=
 
 var_value ::=
 	a_expr
-	| 'ON'
+	| extra_var_value
 
 opt_on_targets_roles ::=
 	'ON' targets_roles
@@ -1421,6 +1421,10 @@ offset_clause ::=
 generic_set ::=
 	var_name to_or_eq var_list
 
+extra_var_value ::=
+	'ON'
+	| cockroachdb_extra_reserved_keyword
+
 targets_roles ::=
 	'ROLE' name_list
 	| targets

pkg/sql/parser/parse_test.go

@@ -1832,6 +1832,9 @@ func TestParse2(t *testing.T) {
 		{`ALTER TABLE a ALTER b TYPE INT`, `ALTER TABLE a ALTER COLUMN b SET DATA TYPE INT`},
 		{`EXPLAIN ANALYZE SELECT 1`, `EXPLAIN ANALYZE (DISTSQL) SELECT 1`},
 
+		{`SET a = INDEX`, `SET a = "index"`},
+		{`SET a = NOTHING`, `SET a = "nothing"`},
+
 		// Regression for #31589
 		{`CREATE TABLE FAMILY (x INT)`,
 			`CREATE TABLE "family" (x INT)`},

pkg/sql/parser/sql.y

@@ -921,7 +921,7 @@ func newNameFromStr(s string) *tree.Name {
 %type <tree.Expr> string_or_placeholder_list
 
 %type <str> unreserved_keyword type_func_name_keyword cockroachdb_extra_type_func_name_keyword
-%type <str> col_name_keyword reserved_keyword cockroachdb_extra_reserved_keyword
+%type <str> col_name_keyword reserved_keyword cockroachdb_extra_reserved_keyword extra_var_value
 
 %type <tree.ConstraintTableDef> table_constraint constraint_elem
 %type <tree.TableDef> index_def
@@ -2823,11 +2823,26 @@ attrs:
 
 var_value:
   a_expr
-| ON
+| extra_var_value
   {
     $$.val = tree.Expr(&tree.UnresolvedName{NumParts: 1, Parts: tree.NameParts{$1}})
   }
 
+// The RHS of a SET statement can contain any valid expression, which
+// themselves can contain identifiers like TRUE, FALSE. These are parsed
+// as column names (via a_expr) and later during semantic analysis
+// assigned their special value.
+//
+// In addition, for compatibility with CockroachDB we need to support
+// the reserved keyword ON (to go along OFF, which is a valid column name).
+//
+// Finally, in PostgreSQL the CockroachDB-reserved words "index",
+// "nothing", etc. are not special and are valid in SET. These need to
+// be allowed here too.
+extra_var_value:
+  ON
+| cockroachdb_extra_reserved_keyword
+
 var_list:
   var_value
   {