mysql性能優化

sql及索引優化

sql查詢優化是平常工做中必不可少的，整體的優化思想是儘量的減小IO操做和記錄掃描行數

開啓慢查詢日誌

查看慢查詢日誌是否打開

mysql> show variables like 'slow_query_log';

+----------------+-------+
| Variable_name | Value |
+----------------+-------+
| slow_query_log | OFF |
+----------------+-------+
1 row in set (0.00 sec)

mysql> set global slow_query_log=on;
Query OK, 0 rows affected (0.05 sec)

慢查詢日誌文件存儲位置

mysql> show variables like '%slow_query_log_file%';
+---------------------+----------------------------------------------+
| Variable_name | Value |
+---------------------+----------------------------------------------+
| slow_query_log_file | C:\xampp\mysql\data\80CEAE742547827-slow.log |
+---------------------+----------------------------------------------+
2 rows in set (0.00 sec)

是否記錄沒有使用索引的sql

mysql> show variables like '%log_queries_not_using%';
+-------------------------------+-------+
| Variable_name | Value |
+-------------------------------+-------+
| log_queries_not_using_indexes | OFF |

+-------------------------------+-------+
1 row in set (0.00 sec)

mysql> set global log_queries_not_using_indexes=on;
Query OK, 0 rows affected (0.00 sec)

執行時間大於N秒的SQL會被記錄

mysql> show variables like '%long_query%';
+-----------------+-----------+
| Variable_name | Value |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
1 row in set (0.00 sec)

mysql> set global long_query_time=1;
Query OK, 0 rows affected (0.00 sec)

max和count語句優化

max和min優化經過對須要求最大值和最小值的字段創建索引，看下面的例子

索引前

mysql> explain select max(payment_date) from payment \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: payment
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 16086 
Extra: NULL
1 row in set (0.00 sec)

mysql> create index ids_payment_date on payment(payment_date);
Query OK, 0 rows affected (0.30 sec)
Records: 0 Duplicates: 0 Warnings: 0

索引後

mysql> explain select max(payment_date) from payment \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: NULL
type: NULL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: NULL
Extra: Select tables optimized away
1 row in set (0.00 sec)

Select tables optimized away表示sql已是最優化了

group by語句優化

優化前

select actor.first_name, actor.last_name, count(*)
from sakila.film_actor
INNER JOIN sakila.actor USING(actor_id)
GROUP BY film_actor.actor_id

 

使用join子查詢的方式優化後，actor表沒有在使用文件排序和臨時表

優化後

select actor.first_name, actor.last_name, c.cnt
from sakila.actor
INNER JOIN (
    select actor_id, count(*) as cnt from sakila.film_actor GROUP BY actor_id
) as c using(actor_id)

雖然優化後仍然掃描了200行的記錄，可是actor表沒有使用文件排序和臨時表

limit查詢優化

優化前

select film_id, description from sakila.film order by film_id limit 50, 5

優化後

select film_id, description from sakila.film where film_id > 50 and film_id <= 55 order by film_id limit 0, 5

從圖中能夠看出掃描記錄數變成了5行，避免數據量大時掃描過多的記錄

這中優化方法有個缺點，當記錄是不連續的狀況時，例如：刪除51，52，53三條記錄，這時50到55之間的記錄就只有兩條了

數據結構優化

系統配置優化

服務器硬件優化