MySQL查询缓存的小知识

--SQL 1
select id, balance from account where id = 121;
--SQL 2
select id, balance From account where id = 121;

#!/usr/bin/perl -w
use Digest::MD5 qw(md5 md5_hex md5_base64);
use Math::BigInt;
my $stmt = "select id, balance from account where id = 121\0"; 
my $hash = md5 $stmt; 
my($a,$b,$msb,$lsb) = unpack("V*",$hash);
my $sqln = $msb*(2**32)+$lsb;
my $stop = log($sqln) / log(32) + 1;
my $sqlid = '';
my $charbase32 = '0123456789abcdfghjkmnpqrstuvwxyz';
my @chars = split '', $charbase32;
for($i=0; $i < $stop-1; $i++){
 my $x = Math::BigInt->new($sqln);
 my $seq = $x->bdiv(32**$i)->bmod(32);
 $sqlid = $chars[$seq].$sqlid;
}
print "SQL is:\n $stmt \nSQL_ID is\n $sqlid\n";

SQL is: select id, balance from account where id = 121 
SQL_ID is dm5c6ck1g7bds
SQL is: select id, balance From account where id = 121 
SQL_ID is 6xb8gvs5cmc9b

mysql> show variables like '%query_cache%';
+------------------------------+----------+
| Variable_name    | Value |
+------------------------------+----------+
| have_query_cache    | YES  |
| query_cache_limit   | 1048576 |
| query_cache_min_res_unit  | 4096  |
| query_cache_size    | 16777216 |
| query_cache_type    | OFF  |
| query_cache_wlock_invalidate | OFF  |

# 修改MySQL配置文件/etc/my.cnf，添加如下配置，重启MySQL server即可。
[mysqld]
query_cache_size = 32M
query_cache_type = 1

--创建一个用户表users，并且插入100w数据。
CREATE TABLE `users` (
 `id` bigint NOT NULL AUTO_INCREMENT,
 `name` varchar(20) NOT NULL DEFAULT '' COMMENT '姓名',
 `age` tinyint NOT NULL DEFAULT '0' COMMENT 'age',
 `gender` char(1) NOT NULL DEFAULT 'M' COMMENT '性别',
 `phone` varchar(16) NOT NULL DEFAULT '' COMMENT '手机号',
 `create_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
 `update_time` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COMMENT='用户信息表';

select count(*) from users;
+----------+
| count(*) |
+----------+
| 1000000 |

#禁用QueryCache的配置
query_cache_size = 0
query_cache_type = 0

--第一次执行查询语句
mysql> select * from users order by create_time desc limit 10;
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| id  | name  | age | gender | phone  | create_time   | update_time   |
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| 997855 | User997854 | 54 | M  | 15240540354 | 2020-12-15 14:34:50 | 2020-12-15 14:34:50 |
.......
10 rows in set (0.89 sec)
--第二次执行同样的查询语句
mysql> select * from users order by create_time desc limit 10;
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| id  | name  | age | gender | phone  | create_time   | update_time   |
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| 997855 | User997854 | 54 | M  | 15240540354 | 2020-12-15 14:34:50 | 2020-12-15 14:34:50 |
.......
10 rows in set (0.90 sec)
-- profile跟踪情况
mysql> show profile cpu,block io for query 1; 
+----------------------+----------+----------+------------+--------------+---------------+
| Status    | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| preparing   | 0.000022 | 0.000017 | 0.000004 |   0 |    0 |
| Sorting result  | 0.000014 | 0.000009 | 0.000005 |   0 |    0 |
| executing   | 0.000011 | 0.000007 | 0.000004 |   0 |    0 |
| Sending data   | 0.000021 | 0.000016 | 0.000004 |   0 |    0 |
| Creating sort index | 0.906290 | 0.826584 | 0.000000 |   0 |    0 |

#禁用QueryCache的配置
query_cache_size = 32M
query_cache_type = 1

--第一次执行查询语句
mysql> select * from users order by create_time desc limit 10;
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| id  | name  | age | gender | phone  | create_time   | update_time   |
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| 997855 | User997854 | 54 | M  | 15240540354 | 2020-12-15 14:34:50 | 2020-12-15 14:34:50 |
.......
10 rows in set (0.89 sec)
--第二次执行查询语句
mysql> select * from users order by create_time desc limit 10;
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| id  | name  | age | gender | phone  | create_time   | update_time   |
+---------+------------+-----+--------+-------------+---------------------+---------------------+
| 997855 | User997854 | 54 | M  | 15240540354 | 2020-12-15 14:34:50 | 2020-12-15 14:34:50 |
.......
10 rows in set (0.00 sec)
-- profile跟踪数据
mysql> show profile cpu,block io for query 3;
+--------------------------------+----------+----------+------------+--------------+---------------+
| Status       | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+--------------------------------+----------+----------+------------+--------------+---------------+
| Waiting for query cache lock | 0.000016 | 0.000015 | 0.000001 |   0 |    0 |
| checking query cache for query | 0.000007 | 0.000007 | 0.000000 |   0 |    0 |
| checking privileges on cached | 0.000004 | 0.000003 | 0.000000 |   0 |    0 |
| checking permissions   | 0.000034 | 0.000033 | 0.000001 |   0 |    0 |
| sending cached result to clien | 0.000018 | 0.000017 | 0.000001 |   0 |    0 |

mysql>SHOW GLOBAL STATUS LIKE 'QCache\_%';
+-------------------------+----------+
| Variable_name   | Value |
+-------------------------+----------+
| Qcache_free_blocks  | 1  | --查询缓存中可用内存块的数目。
| Qcache_free_memory  | 33268592 | --查询缓存的可用内存量。
| Qcache_hits    | 121  | --从QC中获取结果集的次数。
| Qcache_inserts   | 91  | --将查询结果集添加到QC的次数，意味着查询已经不在QC中。
| Qcache_lowmem_prunes | 0  | --由于内存不足而从查询缓存中删除的查询数。
| Qcache_not_cached  | 0  | --未缓存的查询数目。
| Qcache_queries_in_cache | 106  | --在查询缓存中注册的查询数。
| Qcache_total_blocks  | 256  | --查询缓存中的块总数。

           Qcache_hits
Query cache hit rate = ------------------------------------------------ x 100%
      Qcache_hits + Qcache_inserts + Qcache_not_cached
      
        query_cache_size = Qcache_free_memory
Query Cache Avg Query Size = --------------------------------------- 
          Qcache_queries_in_cache