MySQL statement takes more than minute to execute
I am working on a website in which the database is huge. 1 million records are in the table at the time. When I perform a query then it takes too much time to execute. One sample query is given below:
select * from `ratings` order by id limit 499500, 500
Every query takes more than one minute, but when I drop the table to 10 thousand records then this query executes fastly.
As I have read that there is not problem for 1 million records in a table because in database tables, there is not problem of big records.
I have used indexing of id in the table by the help of Stack Overflow question How do I add indices to MySQL tables?, but still I got the same problem.
*** I am using CodeIgniter for the project.
Solution 1:
Note, this is not suggesting for a minute to use MyISAM. I use that only to get my ids, min,max, and count to line up. So ignore the engine, please.
create table ratings
( id int auto_increment primary key,
thing int null
)engine=MyISAM;
insert ratings (thing) values (null),(null),(null),(null),(null),(null),(null),(null),(null);
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
insert ratings (thing) select thing from ratings;
I now have 4.7M rows
select count(*),min(id),max(id) from ratings;
+----------+---------+---------+
| count(*) | min(id) | max(id) |
+----------+---------+---------+
| 4718592 | 1 | 4718592 |
+----------+---------+---------+
select * from `ratings` order by id limit 499500, 500;
-- 1 second on a dumpy laptop
.
explain select * from `ratings` order by id limit 499500, 500;
+----+-------------+---------+------+---------------+------+---------+------+---------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+---------+----------------+
| 1 | SIMPLE | ratings | ALL | NULL | NULL | NULL | NULL | 4718592 | Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+---------+----------------+
.
explain select * from `ratings` where id>=499501 limit 500;
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+
| 1 | SIMPLE | ratings | range | PRIMARY | PRIMARY | 4 | NULL | 4198581 | Using index condition |
+----+-------------+---------+-------+---------------+---------+---------+------+---------+-----------------------+
Moral of the story may be to use a where clause.
One cannot rule out the possibility of a deadlock.
Solution 2:
Start with checking the execution plan of the query to identify the bottleneck and create indexes if required. I think you should atleast have a index on Id column.
There are many factors which can also affect your query performance:
- Fragmentation of Data pages
- Statistics of table not updated
- Many request running in parallel
and many more....
Follow the below links to get the execution plan and to identify the performance degrading factors: http://www.sitepoint.com/using-explain-to-write-better-mysql-queries/
How to optimise MySQL queries based on EXPLAIN plan
Let me know if you face any troubles.