hi~
I test "tablesample" feature on pg9.5 and found that tablesample doesnt support to be applied to subquery.
for example this sql doesnt work on pg9.5:
```
create table t1(
c1 int,
c2 int
);
select c2 from (select * from t1) as t2 tablesample system (50);
```
while i read sql2003 standard about tablesample and think it should allow this usage according to standard.
is it a deviation to sql standard?
