Is there any way to optimize this mysql query? - mysql

Question

i have two tables called hg_questions and hg_tags related with hg_question_tag which has the following structure:
+---------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------------------+------+-----+---------+-------+
| qid | bigint(20) unsigned | YES | | NULL | |
| tagid | bigint(20) unsigned | YES | MUL | NULL | |
| tagname | varchar(50) | YES | | NULL | |
+---------+---------------------+------+-----+---------+-------+
i have only one index on this table for tagid column, the following query runs so slow, since i have exactly 59440 rows for tag number 464 (so many questions in this tag)
SELECT hg_questions.qid,
hg_questions.question,
hg_questions.points,
hg_questions.reward,
hg_questions.answerscount,
hg_questions.created_at,
hg_questions.sections,
hg_questions.answered,
hg_questions.user_id
FROM hg_questions
INNER JOIN hg_question_tag ON hg_question_tag.qid = hg_questions.qid
WHERE hg_question_tag.tagid = 464
ORDER BY points DESC LIMIT 15
OFFSET 0;
when running explain on this query i get:
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+--------+---------------+---------+---------+----------------------------------+-------+----------------------------------------------+
| 1 | SIMPLE | hg_question_tag | ref | tagid | tagid | 9 | const | 59440 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | hg_questions | eq_ref | PRIMARY | PRIMARY | 8 | ejaaba_bilal.hg_question_tag.qid | 1 | |
+----+-------------+-----------------+--------+---------------+---------+---------+----------------------------------+-------+----------------------------------------------+
any ideas how i can optimize this query? or there is a way to get it work faster.
hg_questions has an index on points column
removed order by points makes it work faster like 80%

Answer 1

Here is your query, simplified to some extent:
SELECT q.*
FROM hg_questions q INNER JOIN
hg_question_tag qt
ON qt.qid = q.qid
WHERE qt.tagid = 464
ORDER BY points DESC
LIMIT 15 OFFSET 0;
You are doing both a join and an order by. To optimize this query, try putting an index on hg_question_tag(tagid, qid). This will work if not many questions have the tag.
If many questions have the tag, it might be better to go through the questions and choose the appropriate ones. Try rewriting the query as:
select q.*
from hg_questions q
where exists (select 1 from hg_question_tag qt where qt.qid = q.qid and qt.tagid = 464)
order by points desc
limit 15 offset 0;
Keep the above index and put another index on hg_questions(points, qid).