What index should increase performance of select query? - mysql

Question

This is table structure:
+--------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| visitor_hash | varchar(40) | YES | MUL | NULL | |
| uri | varchar(255) | YES | | NULL | |
| ip_address | char(15) | YES | MUL | NULL | |
| last_visit | datetime | YES | | NULL | |
| visits | int(11) | NO | | NULL | |
| object_app | varchar(255) | YES | MUL | NULL | |
| object_model | varchar(255) | YES | | NULL | |
| object_id | varchar(255) | YES | | NULL | |
| blocked | tinyint(1) | NO | | NULL | |
+--------------+--------------+------+-----+---------+----------------+
This is request:
SELECT `object_id`
FROM `visits_visit`
WHERE `object_model` = 'News'
GROUP BY `object_id`
ORDER BY COUNT( * ) DESC
LIMIT 0, 3
Time for response is ~77,63 ms.
CREATE INDEX resource_model ON visits_visit (object_model(100));
After this request the time for response increased to ~150ms.
How to improve performance for this case? Thank you.
UPDATED:
Answering to Michal Komorowski.
This is explain before index:
+----+-------------+--------------+------+---------------+------+---------+------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+------+---------+------+--------+----------------------------------------------+
| 1 | SIMPLE | visits_visit | ALL | NULL | NULL | NULL | NULL | 142938 | Using where; Using temporary; Using filesort |
+----+-------------+--------------+------+---------------+------+---------+------+--------+----------------------------------------------+
1 row in set (0.00 sec)
And this is after index:
+----+-------------+--------------+------+----------------+----------------+---------+-------+-------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+----------------+----------------+---------+-------+-------+----------------------------------------------+
| 1 | SIMPLE | visits_visit | ref | resource_model | resource_model | 303 | const | 64959 | Using where; Using temporary; Using filesort |
+----+-------------+--------------+------+----------------+----------------+---------+-------+-------+----------------------------------------------+
1 row in set (0.00 sec)
I don't know what gives me this information.
SELECT `object_id`
FROM `visits_visit`
WHERE `object_model` = 'News'
GROUP BY `object_id`
ORDER BY COUNT( * ) DESC
LIMIT 0, 3
78,85 ms before indexing and 365,59 ms after indexing.
Also i have index
CREATE INDEX resource ON visits_visit (object_app(100), object_model(100), object_id(100));
But i need this one, because in other select queries WHERE contains this three keys.
UPDATE:
I'm using django debug toolbar to test performance of requests.
UPDATE:
Query:
ANALYZE TABLE visits_visit;
Output:
+-----------------------------+---------+----------+-----------------------------+
| Table | Op | Msg_type | Msg_text |
+-----------------------------+---------+----------+-----------------------------+
| **************.visits_visit | analyze | status | Table is already up to date |
+-----------------------------+---------+----------+-----------------------------+
1 row in set (0.00 sec)
UPDATE:
SHOW INDEXES FROM visits_visit;
Output:
+--------------+------------+-----------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------------+------------+-----------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| visits_visit | 0 | PRIMARY | 1 | id | A | 142938 | NULL | NULL | | BTREE | | |
| visits_visit | 1 | visits_visit_0880babc | 1 | visitor_hash | A | 142938 | NULL | NULL | YES | BTREE | | |
| visits_visit | 1 | visits_visit_5325a746 | 1 | ip_address | A | 142938 | NULL | NULL | YES | BTREE | | |
| visits_visit | 1 | resource | 1 | object_app | A | 1 | 100 | NULL | YES | BTREE | | |
| visits_visit | 1 | resource | 2 | object_model | A | 3 | 100 | NULL | YES | BTREE | | |
| visits_visit | 1 | resource | 3 | object_id | A | 959 | 100 | NULL | YES | BTREE | | |
+--------------+------------+-----------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

Answer 1

It seems to me that although you have an index, MySQL doesn't know how to use it properly. It happens when information about data distribution (statistics) within a table are not up to date. In order to update them you should call ANALYZE TABLE visits_visit and then check results.

Answer 2

I was confused by misunderstanding of sql mechanisms, so i decided to create model Popular and save instances in it every 24 hours. Thanks to everyone, who tried to help.

Answer 3

As I said in your other question, Prefix indexes are virtually useless; don't use them except in rare circumstances.
Shrink the fields to reasonable lengths and you won't be tempted to use Prefix indexes.
The optimal index for that query is INDEX(object_model, object_id). Attempting to use INDEX(object_model(##), ...) will not get past object_model to anything after it.
If object_model is things like 'News', I suspect the other possible values are short, and perhaps there is a finite number of models. For "short" change to some smaller VARCHAR. For "finite", consider using ENUM('News', 'Weather', 'Sports', ...).
As for why it took longer after indexing...
Without the index, the Optimizer had no choice but to scan the entire table. This is a simple linear scan. It would read but not count any non-News rows.
With the index, the Optimizer has the additional choice of using the index. But, perhaps most rows are News? Well, it would scan the index (nice), but for each News item in the index, it would have to look up the row to get object_id (not so nice). It seems (from the timings) that the latter is less efficient.
By shrinking the declarations and using INDEX(object_model, object_id) (in this order), the query can be performed in the index. Think of the index as a mini-table with just those two columns in it. It is smaller. It is ordered by model, so it only needs to scan the 'News' part. The explain will show this "covering" by saying "Using index".
If all cases, the GROUP BY adds some overhead -- either keeping a hash of object_id in RAM or by saving intermediate results and sorting them. Then the ORDER BY requires a sort (or a priority hash) before the LIMIT can apply.