how to determine slow queries?

Question

My site is experiencing a really slow loading time. I have suspected that it might be javascript or php that causes longer loading time but i have tested my site in YSlow and its grade is B which i think is not bad.

Now i want to check my database if something is wrong with queries, database indexing that causes my site to load slower.

Is there some tutorials or tricks i might read or try to test database to figure out if there is slow queries? Any tips for database management?

Answer 1

I always fall back to 2 mantras for faster query execution -

1. Indexes, indexes and indexes.
2. Try to get rid of JOINS as much as possible.

There are some tried and tested methods to weed out slow queries. You need to turn on slow query log. This logs all those queries which take more than x seconds to execute. x is specified by you in mysql.conf.

Once the slow queries start logging in the log. You can analyse each query using EXPLAIN and appropriately add indexes to speedy the query execution.

Answer 2

I have a thin database abstraction layer on top of PDO (formerly on top of MySQL) that I've baked simple query logging into which I can switch on and off - it allows me to get a report of the queries and how long each one took. Thus rather than having a simple cut-off - something is either a long query or it's not - I get to see all my query times.

MySQL's slow query log is good, but its one-second resolution is not enough for my needs. To me, a lot of the time, a query that takes 200 milliseconds is an indication of something wrong.

I'm showing my age here. After a quick check of the MySQL manual, it turns out that MySQL's long_query_time can be specified down to microsecond resolution since MySQL 5.1.27! Nonetheless, my method is still handy.

Answer 3

Good tutorial for sql performance - focused on MySql. For joins, if you can't get rid of them - use the right one: LEFT JOIN, RIGHT, INNER, OUTER