Using SAMPLE with a WHERE clause

Question

(Oracle 9iR2)
I am using the SAMPLE keyword in a SELECT statement with a WHERE clause. Is the sample extracted before or after applying the WHERE clause?
For example, if I have the statement:
SELECT emp_id FROM employees SAMPLE(10)
WHERE salary > 50000;
Does the database first extract a 10% sample of all the rows and then apply the salary filter on the sample (meaning it's possible that no rows are returned even though there are rows with salary > 50000)? Or does it return a 10% sample of the rows that meet the filter condition (meaning rows will always be returned as long as there are rows with salary > 50000)?

Answer 1

The sample is extracted before the where clause:
SQL> drop table sample_test;
Table dropped.
SQL> create table sample_test (id number, salary number);
Table created.
SQL>
SQL> begin
2 for i in 1..99 loop
3 insert into sample_test values (i, 100);
4 end loop;
5
6 insert into sample_test values (100, 1000);
7 end;
8 /
PL/SQL procedure successfully completed.
SQL>
SQL> commit;
Commit complete.
SQL>
SQL> select * from sample_test sample(5)
2 where salary > 100;
no rows selected