Optimize "JOIN" query - mysql

Question

this is my query from my source code
SELECT `truyen`.*, MAX(chapter.chapter) AS last_chapter
FROM (`truyen`)
LEFT JOIN `chapter` ON `chapter`.`truyen` = `truyen`.`Id`
WHERE `truyen`.`title` LIKE \'%%\'
GROUP BY `truyen`.`Id`
LIMIT 250
When I install it on iFastnet host, It cause over 500,000 rows to be examined due to the join, and the query is being blocked (this would used over 100% of a CPU, which ultimately would cause server instability).
I also tried to add this line before the query, it fixed the problem above but lead to another issue making some of functions can not run correctly
mysql_query("SET SQL_BIG_SELECTS=1");
How can I fix this problem without buying another hosting ?
Thanks.

Answer 1

You might be looking for an INNER JOIN. That would remove results that do not match. I find INNER JOINs to be faster than LEFT JOINs.
However, I'm not sure what results you are actually looking for. But because you are using the GROUP BY, it looks like the INNER JOIN might work for you.
One thing I would recommend is copy and paste the query that it generates into SQL with DESCRIBE before it.
So if the query ended up being:
SELECT truyen.*, MAX(chapter.chapter) AS last_chapter FROM truyen
LEFT JOIN chapter ON chapter.truyen = truyen.Id
WHERE truyen.title LIKE '%queryString%'
You would type:
DESCRIBE SELECT truyen.*, MAX(chapter.chapter) AS last_chapter FROM truyen
LEFT JOIN chapter ON chapter.truyen = truyen.Id
WHERE truyen.title LIKE '%queryString%'
This will tell you if you could possibly ad an index to your table to JOIN on faster.
I hope this at least points you in the right direction.
Michael Berkowski seems to agree with the indexing, which you will be able to see from the DESCRIBE.

Answer 2

Please look if you have indexes on chapter.chapter and chapter.truyen. If not, set them and try again. If this is not successful try these suggestions:
Do you have the possibility to flag permanently on insert/update your last chapter in a column of your chapter table? Then you could use it to reduce the joined rows and you could drop out the GROUP BY. Maybe in this way:
SELECT `truyen`.*, `chapter`.`chapter` as `last_chapter`
FROM `truyen`, `chapter`
WHERE `chapter`.`truyen` = `truyen`.`Id`
AND `chapter`.`flag_last_chapter` = 1
AND `truyen`.`title` LIKE '%queryString%'
LIMIT 250
Or create a new table for that instead:
INSERT INTO new_table (truyen, last_chapter)
SELECT truyen, MAX(chapter) FROM chapter GROUP BY truyen;
SELECT `truyen`.*, `new_table`.`last_chapter`
FROM (`truyen`)
LEFT JOIN `new_table` ON `new_table`.`truyen` = `truyen`.`Id`
WHERE `truyen`.`title` LIKE '%queryString%'
GROUP BY `truyen`.`Id`
LIMIT 250
Otherwise you could just fetch the 250 rows of truyen, collect your truyen ids in an array and build another SQL Statement to select the 250 rows of the chapter table. I have seen in your original question that you can use PHP for that. So you could merge the results after that:
SELECT * FROM truyen
WHERE title LIKE '%queryString%'
LIMIT 250
SELECT truyen, MAX(chapter) AS last_chapter
FROM chapter
WHERE truyen in (comma_separated_ids_from_first_select)