MySQL 8.0 information_schema系统库的改进

information_schema有何用？
mysql8.0 之前的查询方式
mysql8.0 开始的查询方式
测试5.7和8.0不同版本访问I_S库的性能
结论

information_schema有何用？

通过I_S获取mysql的一些元数据信息
获取表的数据文件、索引文件的大小、碎片情况、表行数、自增列增长情况等
获取正在运行的事务有那些，是否有阻塞等
获取当前mysql的连接processlist等等

mysql8.0 之前的查询方式

会在查询information_schema 某个表时创建临时表

来自文件的元数据，扫描文件系统获取FRM文件的表定义
存储引擎的详细信息，例如动态表统计信息
来自MySQL服务器中全局数据结构的数据

在表的数量很多时，每次查询I_S会从文件系统中读取每个单独的FRM文件，使用更多的CPU周期来打开表并准备相关的内存数据结构

mysql8.0 开始的查询方式

引入了基于InnoDB的本地数据字典表
表中不在有FRM表定义文件
所有数据库表的元数据都存储在事务数据字典表中
I_S中表被设计为数据字典表上的VIEW（有些还是临时表0_0）

消除了以下成本

查询INFORMATION_SCHEMA时创建的临时表

扫描文件系统目录以查找FRM文件

改进

利用MySQL优化器的全部功能，使用数据字典表上的索引来更好的查询

mysql5.7中表文件

ll test*

Jul 10 10:52 testse.frm

Jul 10 10:52 testse.ibd

mysql8.0中表文件

ll test*

Jul 10 10:25 testse.ibd

mysql5.7 和mysql8.0 I_S 中tables 表的存在方式

mysql5.7.22
show create table information_schema.tables\G
*************************** 1. row ***************************
       Table: TABLES
Create Table: CREATE TEMPORARY TABLE `TABLES` (
  `TABLE_CATALOG` varchar(512) NOT NULL DEFAULT '',
  `TABLE_SCHEMA` varchar(64) NOT NULL DEFAULT '',
  `TABLE_NAME` varchar(64) NOT NULL DEFAULT '',
  `TABLE_TYPE` varchar(64) NOT NULL DEFAULT '',
  `ENGINE` varchar(64) DEFAULT NULL,
mysql8.0.15
  show create table information_schema.tables\G
*************************** 1. row ***************************
                View: TABLES
         Create View: CREATE ALGORITHM=UNDEFINED DEFINER=`mysql.infoschema`@`localhost` SQL SECURITY DEFINER VIEW `

mysql5.7 中I_S中tables表是以临时表的形式存在的（查询该表就会创建临时表，创建的临时表过多，可能会导致mysql占用的内存暴涨，出现OOM）
mysql8.0 中I_S中tables表以视图的形式存在（查询该视图，不会创建临时表，会使用到视图中表的索引）

mysql5.7中获取表大小情况

SELECT   table_name,   CONCAT(FORMAT(data_length / 1024 / 1024, 2),   'M') AS dbdata_size,   CONCAT(FORMAT(index_length / 1024 / 1024, 2),   'M') AS dbindex_size,   CONCAT(     FORMAT((data_length + index_length) / 1024 / 1024 / 1024,2),   'G') AS `db_size(G)`,   AVG_ROW_LENGTH,   table_rows,   update_time FROM   information_schema.tables WHERE table_schema = 'test'   AND table_name = 'testse';
+------------+-------------+--------------+------------+----------------+------------+---------------------+
| table_name | dbdata_size | dbindex_size | db_size(G) | AVG_ROW_LENGTH | table_rows | update_time         |
+------------+-------------+--------------+------------+----------------+------------+---------------------+
| testse     | 0.02M       | 0.02M        | 0.00G      |            862 |         19 | 2019-07-10 10:52:02 |
+------------+-------------+--------------+------------+----------------+------------+---------------------+
执行计划中出现了where，未用到索引（存储引擎检索数据后，server层进行过滤）
desc SELECT   table_name,   CONCAT(FORMAT(data_length / 1024 / 1024, 2),   'M') AS dbdata_size,   CONCAT(FORMAT(index_length / 1024 / 1024, 2),   'M') AS dbindex_size,   CONCAT(     FORMAT(data_length + index_length / 1024 / 1024 / 1024,     2   ),   'G') AS `db_size(G)`,   AVG_ROW_LENGTH,   table_rows,   update_time FROM   information_schema.tables WHERE table_schema = 'test'   AND table_name = 'testse';
+----+-------------+--------+------------+------+---------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
| id | select_type | table  | partitions | type | possible_keys | key                     | key_len | ref  | rows | filtered | Extra                                             |
+----+-------------+--------+------------+------+---------------+-------------------------+---------+------+------+----------+---------------------------------------------------+
|  1 | SIMPLE      | tables | NULL       | ALL  | NULL          | TABLE_SCHEMA,TABLE_NAME | NULL    | NULL | NULL |     NULL | Using where; Open_full_table; Scanned 0 databases |
+----+-------------+--------+------------+------+---------------+-------------------------+---------+------+------+----------+---------------------------------------------------+

mysql8.0中获取表大小情况

SELECT   table_name,   CONCAT(FORMAT(data_length / 1024 / 1024, 2),   'M') AS dbdata_size,   CONCAT(FORMAT(index_length / 1024 / 1024, 2),   'M') AS dbindex_size,   CONCAT(     FORMAT((data_length + index_length) / 1024 / 1024 / 1024,2),   'G') AS `db_size(G)`,   AVG_ROW_LENGTH,   table_rows,   update_time FROM   information_schema.tables WHERE table_schema = 'test'   AND table_name = 'testse';
+------------+-------------+--------------+------------+----------------+------------+---------------------+
| TABLE_NAME | dbdata_size | dbindex_size | db_size(G) | AVG_ROW_LENGTH | TABLE_ROWS | UPDATE_TIME         |
+------------+-------------+--------------+------------+----------------+------------+---------------------+
| testse     | 0.02M       | 0.02M        | 0.00G      |            862 |         19 | 2019-07-10 10:25:16 |
+------------+-------------+--------------+------------+----------------+------------+---------------------+
能使用到数据字典表的索引
desc SELECT   table_name,   CONCAT(FORMAT(data_length / 1024 / 1024, 2),   'M') AS dbdata_size,   CONCAT(FORMAT(index_length / 1024 / 1024, 2),   'M') AS dbindex_size,   CONCAT(     FORMAT((data_length + index_length) / 1024 / 1024 / 1024,2),   'G') AS `db_size(G)`,   AVG_ROW_LENGTH,   table_rows,   update_time FROM   information_schema.tables WHERE table_schema = 'test'   AND table_name = 'testse';
+----+-------------+-------+------------+--------+--------------------+------------+---------+-------------------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys      | key        | key_len | ref                     | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+--------------------+------------+---------+-------------------------+------+----------+-------------+
|  1 | SIMPLE      | cat   | NULL       | index  | PRIMARY            | name       | 194     | NULL                    |    1 |   100.00 | Using index |
|  1 | SIMPLE      | sch   | NULL       | eq_ref | PRIMARY,catalog_id | catalog_id | 202     | mysql.cat.id,const      |    1 |   100.00 | Using index |
|  1 | SIMPLE      | tbl   | NULL       | eq_ref | schema_id          | schema_id  | 202     | mysql.sch.id,const      |    1 |   100.00 | Using where |
|  1 | SIMPLE      | stat  | NULL       | const  | PRIMARY            | PRIMARY    | 388     | const,const             |    1 |   100.00 | NULL        |
|  1 | SIMPLE      | ts    | NULL       | eq_ref | PRIMARY            | PRIMARY    | 8       | mysql.tbl.tablespace_id |    1 |   100.00 | NULL        |
|  1 | SIMPLE      | col   | NULL       | eq_ref | PRIMARY            | PRIMARY    | 8       | mysql.tbl.collation_id  |    1 |   100.00 | Using index |
+----+-------------+-------+------------+--------+--------------------+------------+---------+-------------------------+------+----------+-------------+

测试5.7和8.0不同版本访问I_S库的性能

机器

cat /etc/redhat-release | xargs echo '版本 ' && dmidecode -s system-product-name | xargs echo '是否虚拟化 ' && cat /proc/cpuinfo |grep "processor"|wc -l | xargs echo 'cpu核数 '
版本  CentOS Linux release 7.5.1804 (Core)
是否虚拟化  KVM
cpu核数  4

1、分别在mysql5.7和mysql8.0 中创建5个库，每个库中30个表（共1500个表），每个表记录数为10000

user=admin
passwd=admin
port=57222
host=127.0.0.1
#创建5个库,分别在每个库中创建30个表
for i in {1..5};do
mysql -u$user -p$passwd -P$port -h$host<<EOF
  create database if not exists test_${i};
EOF
sysbench /usr/local/share/sysbench/oltp_read_write.lua --mysql_storage_engine=innodb  --table-size=10000 --tables=30 --mysql-db=test_${i} --mysql-user=$user --mysql-password=$passwd --mysql-port=$port  --mysql-host=$host --threads=8 --time=10 --report-interval=1 --events=0 --db-driver=mysql prepare
done;

2、自定义访问I_S.tables表的sysbench脚本

cat tests/mytest.lua

require("oltp_common")
function thread_init(thread_id)
 drv=sysbench.sql.driver()
 con=drv:connect()
end
local function get_rand_db()
   return sysbench.rand.uniform(1, 5)
end
function event(thread_id)
local vid1
local dbprefix
vid1=get_rand_db()
dbprefix = "test_"
-- 生成5个db中的随机一个db,如 test_1 ,test_5
-- vid2=string.format("'%s%s'",test_,vid1)
vid2="'" .. dbprefix .. vid1 .. "'"
con:query("SELECT   table_name,   CONCAT(FORMAT(data_length / 1024 / 1024, 2),   'M') AS dbdata_size,   CONCAT(FORMAT(index_length / 1024 / 1024, 2),   'M') AS dbindex_size,   CONCAT(     FORMAT((data_length + index_length) / 1024 / 1024 / 1024,2),   'G') AS `db_size(G)`,   AVG_ROW_LENGTH,   table_rows,   update_time FROM   information_schema.tables WHERE table_schema =" .. vid2)
end
function thread_done()
 con:disconnect()
end

3、脚本通过sysbench测试访问I_S.tables的qps

cat test_I_S_access.sh

user=admin
passwd=admin
host=127.0.0.1
#输入mysql端口参数
port=$1
# port=8015
#输入sysbench测试时间参数多少秒
run_time=$2
function get_create_tmp_tables(){
mysql -u$user -p$passwd -P$port -h$host<<EOF
select @@version as 'mysql version';
show global  status like '%tmp_table%';
EOF
} 2>/dev/null
function begin_test(){
  cd /usr/local/share/sysbench
   sysbench ./tests/mytest.lua --mysql-db=test_1  --mysql-host=$host --mysql-port=$port --mysql-user=$user --mysql-password=$passwd  --threads=40  --report-interval=10 --rand-type=uniform --time=$run_time    run
}
service mysqld${port} restart
get_create_tmp_tables
begin_test
get_create_tmp_tables

4、mysql5.7和mysql8.0 主要配置

mysql5.7.22
5.7.22-log
innodb_buffer_pool_size 128M
innodb_log_buffer_size  64M
innodb_log_file_size    48M
binlog_format   ROW
innodb_flush_log_at_trx_commit  1
sync_binlog     1
mysql8.0.15
8.0.15
innodb_buffer_pool_size 128M
innodb_log_buffer_size  64M
innodb_log_file_size    48M
binlog_format   ROW
innodb_flush_log_at_trx_commit  1
sync_binlog     1

5、mysql5.7开启测试

bash test_I_S_acess.sh 57222 60

Shutting down MySQL.. SUCCESS!
Starting MySQL.. SUCCESS!
mysql version
5.7.22-log
Variable_name   Value
Created_tmp_tables      1
sysbench 1.1.0 (using bundled LuaJIT 2.1.0-beta3)
Running the test with following options:
Number of threads: 40
Report intermediate results every 10 second(s)
Initializing random number generator from current time
Initializing worker threads...
Threads started!
[ 10s ] thds: 40 tps: 4765.73 qps: 4765.73 (r/w/o: 4765.73/0.00/0.00) lat (ms,95%): 18.95 err/s: 0.00 reconn/s:                                                                                                              0.00
[ 20s ] thds: 40 tps: 5409.00 qps: 5409.00 (r/w/o: 5409.00/0.00/0.00) lat (ms,95%): 17.95 err/s: 0.00 reconn/s:                                                                                                              0.00
[ 30s ] thds: 40 tps: 5154.45 qps: 5154.45 (r/w/o: 5154.45/0.00/0.00) lat (ms,95%): 18.61 err/s: 0.00 reconn/s:                                                                                                              0.00
[ 40s ] thds: 40 tps: 5383.50 qps: 5383.50 (r/w/o: 5383.50/0.00/0.00) lat (ms,95%): 17.63 err/s: 0.00 reconn/s:                                                                                                              0.00
[ 50s ] thds: 40 tps: 5456.11 qps: 5456.11 (r/w/o: 5456.11/0.00/0.00) lat (ms,95%): 17.63 err/s: 0.00 reconn/s:                                                                                                              0.00
[ 60s ] thds: 40 tps: 5458.66 qps: 5458.66 (r/w/o: 5458.66/0.00/0.00) lat (ms,95%): 17.63 err/s: 0.00 reconn/s:                                                                                                              0.00
SQL statistics:
    queries performed:
        read:                            316322
        write:                           0
        other:                           0
        total:                           316322
    transactions:                        316322 (5270.99 per sec.)
    queries:                             316322 (5270.99 per sec.)
    ignored errors:                      0      (0.00 per sec.)
    reconnects:                          0      (0.00 per sec.)
Throughput:
    events/s (eps):                      5270.9892
    time elapsed:                        60.0119s
    total number of events:              316322
Latency (ms):
         min:                                    0.42
         avg:                                    7.59
         max:                                  858.18
         95th percentile:                       17.95
         sum:                              2399738.58
Threads fairness:
    events (avg/stddev):           7908.0500/53.41
    execution time (avg/stddev):   59.9935/0.00
mysql version
5.7.22-log
Variable_name   Value
Created_tmp_tables      316327

cpu使用率接近100%

mysql5.7.22 40个并发线程压力测试60秒，访问I_S.tables 平均qps为5k，并创建了31w个临时表!

6、mysql8.0开启测试

Shutting down MySQL.. SUCCESS!
Starting MySQL.... SUCCESS!
mysql version
8.0.15
Variable_name   Value
Created_tmp_tables      1
sysbench 1.1.0 (using bundled LuaJIT 2.1.0-beta3)
Running the test with following options:
Number of threads: 40
Report intermediate results every 10 second(s)
Initializing random number generator from current time
Initializing worker threads...
Threads started!
[ 10s ] thds: 40 tps: 1283.76 qps: 1283.76 (r/w/o: 1283.76/0.00/0.00) lat (ms,95%): 41.10 err/s: 0.00 reconn/s: 0.00
[ 20s ] thds: 40 tps: 1286.43 qps: 1286.43 (r/w/o: 1286.43/0.00/0.00) lat (ms,95%): 39.65 err/s: 0.00 reconn/s: 0.00
[ 30s ] thds: 40 tps: 1333.00 qps: 1333.00 (r/w/o: 1333.00/0.00/0.00) lat (ms,95%): 37.56 err/s: 0.00 reconn/s: 0.00
[ 40s ] thds: 40 tps: 1280.79 qps: 1280.79 (r/w/o: 1280.79/0.00/0.00) lat (ms,95%): 40.37 err/s: 0.00 reconn/s: 0.00
[ 50s ] thds: 40 tps: 1313.57 qps: 1313.57 (r/w/o: 1313.57/0.00/0.00) lat (ms,95%): 38.94 err/s: 0.00 reconn/s: 0.00
[ 60s ] thds: 40 tps: 1222.95 qps: 1222.95 (r/w/o: 1222.95/0.00/0.00) lat (ms,95%): 42.61 err/s: 0.00 reconn/s: 0.00
SQL statistics:
    queries performed:
        read:                            77248
        write:                           0
        other:                           0
        total:                           77248
    transactions:                        77248  (1286.95 per sec.)
    queries:                             77248  (1286.95 per sec.)
    ignored errors:                      0      (0.00 per sec.)
    reconnects:                          0      (0.00 per sec.)
Throughput:
    events/s (eps):                      1286.9486
    time elapsed:                        60.0242s
    total number of events:              77248
Latency (ms):
         min:                                    1.88
         avg:                                   31.08
         max:                                  692.98
         95th percentile:                       40.37
         sum:                              2400505.33
Threads fairness:
    events (avg/stddev):           1931.2000/51.17
    execution time (avg/stddev):   60.0126/0.00
mysql version
8.0.15
Variable_name   Value
Created_tmp_tables      2

cpu使用率接近100%

mysql8.0.15 40个并发线程，压力测试60秒，访问I_S.tables 平均qps为1.2k，并创建了1个临时表!

结论

mysql8.0开始查询I_S中表不会再从文件系统(FRM, TRG，OPT文件)和myisam、innodb系统表读取元数据信息（8.0开始元数据信息统一存放在数据字典表中）
mysql8.0 访问I_S.tables不会创建临时表，这减少了内存暴涨的可能，但访问I_S.tables的qps大约是mysql5.7.22的1/5，访问速度没有mysql5.7.22的快
mysql8.0 访问I_S.tables 用户空间cpu消耗没有mysql5.7的高（没有创建临时表的缘故吧），但系统空间cpu消耗高于mysql5.7！

参考

MySQL 8.0: Improvements to Information_schema