mysql之分页与慢日志以及表知识补充

1.分页.
    select * from tb1 limit 100000000,10;
    提高分页的速率:
     加上条件:使它跳过 100000000 条
     如:将当前页的最后的nid 用全局变量记录(下一页)
        将当前页的第一个nid 用全局变量记录(上一页)
2.慢日志
默认情况下,mysql的慢日志功能是关闭的
如果要启用,要去内存中开启
mysql> show variables like '%query%';
+------------------------------+----------------------------------------------------------------+
| Variable_name                | Value                                                          |
+------------------------------+----------------------------------------------------------------+
| binlog_rows_query_log_events | OFF                                                            |
| ft_query_expansion_limit     | 20                                                             |
| have_query_cache             | NO                                                             |
| long_query_time              | 10.000000                                                      |
| query_alloc_block_size       | 8192                                                           |
| query_prealloc_size          | 8192                                                           |
| slow_query_log               | OFF                                                            |
| slow_query_log_file          | D:mysql_lovmysql-8.0.19-winx64dataDESKTOP-4FRVIM6-slow.log |
+------------------------------+----------------------------------------------------------------+
8 rows in set, 1 warning (0.04 sec)

mysql> set long_query_time=3;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%query%';
+------------------------------+----------------------------------------------------------------+
| Variable_name                | Value                                                          |
+------------------------------+----------------------------------------------------------------+
| binlog_rows_query_log_events | OFF                                                            |
| ft_query_expansion_limit     | 20                                                             |
| have_query_cache             | NO                                                             |
| long_query_time              | 3.000000                                                       |
| query_alloc_block_size       | 8192                                                           |
| query_prealloc_size          | 8192                                                           |
| slow_query_log               | OFF                                                            |
| slow_query_log_file          | D:mysql_lovmysql-8.0.19-winx64dataDESKTOP-4FRVIM6-slow.log |
+------------------------------+----------------------------------------------------------------+
8 rows in set, 1 warning (0.00 sec)

3.表增加，删除，修改 列

3.1.给一个表添加列:---  alter table 表名 add 列名 列类型;
    mysql> alter table tb3 add num int;
    Query OK, 0 rows affected (0.02 sec)
    Records: 0  Duplicates: 0  Warnings: 0
3.2.删除一列: --- alter table 表名 drop column 列名;
    mysql> alter table tb3 drop column num;
    Query OK, 0 rows affected (0.04 sec)
    Records: 0  Duplicates: 0  Warnings: 0
3.3.修改列 :alter table 表名 modify 列名 列类型;
    mysql> alter table tb3 add child char(255);
    Query OK, 0 rows affected (0.02 sec)
    Records: 0  Duplicates: 0  Warnings: 0
    修改如下:
    mysql> alter table tb3 modify child varchar(255);
    Query OK, 0 rows affected (0.05 sec)
    Records: 0  Duplicates: 0  Warnings: 0