support union stmt in expr

support array index, == in hive

support exec in tsql, cross join in hive

support tablesample in pg, fix cast bug in pg, support rlike in hive

fix on clause bug in pg, support row in windows function in mysql, ca…

…st with expr in pg

fix many bugs, support window in pg and mysql

move unay expr after or expr

support function expression as table in pg, support convert and cast …

…in mysql

support where clause wrapped by parentheses, fix schema prefix bug in pg

support schema prefix in pg, sqlify replace into ast, fix string cons…

…tants bug in pg