SQL: How to decrease the statement execution time? - mysql

Question

I'm not an expert in SQL, i have an sql statement :
SELECT * FROM articles WHERE article_id IN
(SELECT distinct(content_id) FROM contents_by_cats WHERE cat_id='$cat')
AND permission='true' AND date <= '$now_date_time' ORDER BY date DESC;
Table contents_by_cats has 11000 rows.
Table articles has 2700 rows.
Variables $now_date_time and $cat are php variables.
This query takes about 10 seconds to return the values (i think because it has nested SELECT statements) , and 10 seconds is a big amount of time.
How can i achieve this in another way ? (Views or JOIN) ?
I think JOIN will help me here but i don't know how to use it properly for the SQL statement that i mentioned.
Thanks in advance.

Answer 1

A JOIN is exactly what you are looking for. Try something like this:
SELECT DISTINCT articles.*
FROM articles
JOIN contents_by_cats ON articles.article_id = contents_by_cats.content_id
WHERE contents_by_cats.cat_id='$cat'
AND articles.permission='true'
AND articles.date <= '$now_date_time'
ORDER BY date DESC;
If your query is still not as fast as you would like then check that you have an index on articles.article_id and contents_by_cats.content_id and contents_by_cats.cat_id. Depending on the data you may want an index on articles.date as well.
Do note that if the $cat and $now_date_time values are coming from a user then you should really be preparing and binding the query rather than just dumping these values into the query.

Answer 2

This is the query we are starting with:
SELECT a.*
FROM articles a
WHERE article_id IN (SELECT distinct(content_id)
FROM contents_by_cats
WHERE cat_id ='$cat'
) AND
permission ='true' AND
date <= '$now_date_time'
ORDER BY date DESC;
Two things will help this query. The first is to rewrite it using exists rather than in and to simplify the subquery:
SELECT a.*
FROM articles a
WHERE EXISTS (SELECT 1
FROM contents_by_cats cbc
WHERE cbc.content_id = a.article_id and cat_id = '$cat'
) AND
permission ='true' AND
date <= '$now_date_time'
ORDER BY date DESC;
Second, you want indexes on both articles and contents_by_cats:
create index idx_articles_3 on articles(permission, date, article_id);
create index idx_contents_by_cats_2 on contents_by_cat(content_id, cat_id);
By the way, instead of $now_date_time, you can just use the now() function in MySQL.