How to speed up this oracle select query? - oracle

Question

I have following sql query in code, which is makes query go slow:
SELECT a.*,
(SELECT CASE
WHEN (score IS NOT NULL OR comments IS NOT NULL)
THEN
' ( score : ' || TO_CHAR (SCORE) || ' )'
ELSE
' ( hələ )'
END
FROM t_lecture_task_present b
WHERE b.t_idx = a.t_idx AND B.STUDENT_ID = '{$member_code}')
AS task_score
FROM t_lecture_task a
WHERE a.lec_open_idx = '24422'
ORDER BY s_date ASC, t_idx ASC
(16 seconds)
If I try query without
(SELECT CASE
WHEN (score IS NOT NULL OR comments IS NOT NULL)
THEN
' ( score : ' || TO_CHAR (SCORE) || ' )'
ELSE
' ( hələ )'
END
FROM t_lecture_task_present b
WHERE b.t_idx = a.t_idx AND B.STUDENT_ID = '{$member_code}')
as task_score
it works faster.
So, I tried removing or comments is not null, and it works got 2 seconds faster.

Answer 1

You can't compare the performance of queries that produce different results :)
Depending on data distribution in your tables, you would likely benefit from these indexes:
t_lecture_task(lec_open_idx)
t_lecture_task_present(t_idx, student_id)
Try to re-write your query to use joins instead of scalar sub queries (select as a column). Not only are they more awkward to read, they are more difficult to optimize.
select a.*
,case when score is not null
or comments is not null then ' ( score : ' || to_char (score) || ' )'
else ' ( hələ )'
end as task_score
from t_lecture_task a
left join t_lecture_task_present b on(
b.t_idx = a.t_idx
and b.student_id = '{$member_code}')
)
where a.lec_open_idx = '24422';