Fixed bug#18175: The nest_level counter wasn't decremented for union parts which

resulted in a wrong error message.

The nest_level counter indicates the depth of nesting for a subselect. It is
needed to properly resolve aggregate functions in nested subselects. Obviously 
it shouldn't be incremented for UNION parts because they have the same level of
nesting. This counter was incremented by 1 in the mysql_new_select() function
for any new select and wasn't decremented for UNION parts. This resulted in
wrongly reported error messages.

Now the nest_level counter is decremented by 1 for any union part.

mysql-test/r/union.result

@@ -1306,3 +1306,48 @@ id
 5
 99
 drop table t1;
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1));
+avg(1)
+NULL

mysql-test/t/union.test

@@ -793,3 +793,51 @@ select id from t1 union all select 99 order by 1;
 drop table t1;
 
 # End of 4.1 tests
+
+#
+# Bug#18175: Union select over 129 tables with a sum function fails.
+#
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1)) union
+(select avg(1)) union (select avg(1)) union (select avg(1));
+

sql/sql_yacc.yy

@@ -8752,6 +8752,8 @@ union_list:
             yyerror(ER(ER_SYNTAX_ERROR));
 	    YYABORT;
 	  }
+          /* This counter shouldn't be incremented for UNION parts */
+          Lex->nest_level--;
 	  if (mysql_new_select(lex, 0))
 	    YYABORT;
           mysql_init_select(lex);