Mysql Select INNER JOIN with order by very slow

Question

I'm trying to speed up a mysql query. The Listings table has several million rows. If I don't sort them later I get the result in 0.1 seconds but once I sort it takes 7 seconds. What can I improve to speed up the query?

SELECT l.* 
FROM listings l 
INNER JOIN listings_categories lc 
ON l.id=lc.list_id 
AND lc.cat_id='2058' 
INNER JOIN locations loc 
ON l.location_id=loc.id 
WHERE l.location_id 
IN (7841,7842,7843,7844,7845,7846,7847,7848,7849,7850,7851,7852,7853,7854,7855,7856,7857,7858,7859,7860,7861,7862,7863,7864,7865,7866,7867,7868,7869,7870,7871,7872,7873,7874,7875,7876,7877,7878,7879,7880,7881,7882,7883,7884,7885,7886,7887,7888,7889,7890,7891,7892,7893,7894,7895,7896,7897,7898,7899,7900,7901,7902,7903) 
ORDER BY date 
DESC LIMIT 0,10;

EXPLAIN SELECT: Using Index l=date, loc=primary, lc=primary

Answer 1

Such performance questions are really difficult to answer and depend on the setup, indexes etc. So, there will likely not the one and only solution and even not really correct or incorrect attempts to improve the speed. This is a lof of try and error. Anyway, some points I noted which often cause performance issues are:

• Avoid conditions within joins that should be placed in the where instead. A join should contain the columns only that will be joined, no further conditions. So the "lc.cat_id='2058" should be put in the where clause.
• Using IN is often slow. You could try to replace it by using OR (l.location_id = 7841 OR location_id = 7842 OR...)
• Open the query execution plan and check whether there is something useful for you.
• Try to find out if there are special cases/values within the affected columns which slow down your query
• Change "ORDER BY date" to "ORDER BY tablealias.date" and check if this makes a difference in performance. Even if not, it is better to read.
• If you can rename the column "date", do this because using SQL keywords as table name or column name is no good idea. I'm unsure if this influences the performance, but it should be avoided if possible.

Good luck!