Question

When joining across tables (as in the examples below), is there an efficiency difference between joining on the tables or joining subqueries containing only the needed columns?

In other words, is there a difference in efficiency between these two tables?

SELECT result
  FROM result_tbl
  JOIN test_tbl                    USING (test_id)
  JOIN sample_tbl                  USING (sample_id)
  JOIN (SELECT request_id
          FROM request_tbl
         WHERE request_status='A') USING(request_id)

vs

SELECT result
  FROM (SELECT result,  test_id   FROM result_tbl)
  JOIN (SELECT test_id, sample_id FROM test_tbl)   USING(test_id)
  JOIN (SELECT sample_id          FROM sample_tbl) USING(sample_id)
  JOIN (SELECT request_id
          FROM request_tbl
         WHERE request_status='A')                 USING(request_id)

Answer 1

It doesn’t matter. It may actually be WORSE since you are taking control away from the optimizer which generally knows best.

However, remember if you are doing a JOIN and only including a column from one of the tables that it is QUITE OFTEN better to re-write it as a series of EXISTS statements — because that’s what you really mean. JOINs (with some exceptions) will join matching rows which is a lot more work for the optimizer to do.

e.g.

SELECT t1.id1
  FROM table1 t1
 INNER JOIN table2 ON something = something

should almost always be

SELECT id1
  FROM table1 t1
 WHERE EXISTS( SELECT *
                 FROM table2
                WHERE something = something )

For simple queries the optimizer may reduce the query plans into identical ones. Check it out on your DBMS.

Also this is a code smell and probably should be changed:

JOIN (SELECT request_id
FROM request_tbl
WHERE request_status=’A’)

to

SELECT result
  FROM request
 WHERE EXISTS(...)
   AND request_status = 'A'