> Now I want to get sequence fields following:
>
> test=# select * from (select relname from pg_class where relkind='S') as foo;
>
> My Result:
>
> relname
> -------------------------
> test_seq
> (1 rows)
>
> Why? FROM Clause subquery not working? or my mistakes?
That's not what the above does. The subselect in from isn't a substitute
for table names, it's a substitute for a table. Think of it almost as an
inplace view definition.
