SQL optimization for large data_sets - sql

Question

i got some nasty sql performance issue. I need to execute statment like:
SELECT *
FROM (SELECT /*+ FIRST_ROWS(26) */
a.*, ROWNUM rnum
FROM (SELECT *
FROM t1
WHERE t1_col1 = 'val1'
AND g_dom in ('1', '2', '3')
AND g_context IN ('3', '4', '5', '6')
AND i_col = 1
AND f_col in ('1', '2', '3', '4')
AND e_g IN (SELECT e_g
FROM t2
WHERE t2_col1 = 'val1'
AND g_context IN ('3', '4', '5', '6')
AND val like 'some val%')
ORDER BY order_id DESC) a)
WHERE rnum > 0;
Basically we got table t1 (our data table), and t2 (our support values). We got 1kk records in t1 and 10kk in t2. Column g_context narrows our data sets, but still, val had something like 500k records. We need 25 rows ordered by order_id.
Is there any way to tell inner statement
SELECT e_g FROM t2 WHERE t2_col1='val1' AND g_context IN('3','4','5','6' ) AND val like 'some val%
to get only 25 records that's match out outer statement criteria ?

Answer 1

Why not move rownum and the hint into the inner query like so:
SELECT t1.*,row_number() over (order by order_id desc) rn /*+ FIRST_ROWS(26) */
FROM t1
WHERE t1_col1 = 'val1'
AND g_dom in ('1', '2', '3')
AND g_context IN ('3', '4', '5', '6')
AND i_col = 1
AND f_col in ('1', '2', '3', '4')
AND e_g IN (SELECT e_g
FROM t2
WHERE t2_col1 = 'val1'
AND g_context IN ('3', '4', '5', '6')
AND val like 'some val%')
ORDER BY order_id DESC
Too me this extra subselect with hint and rownum does not seem to make any sense.
And the where-clause should be "WHERE rnum < 26", shouldn't it?