oracle如何查看当前有哪些用户连接到数据库

可以执行以下语句：
select username,serial#, sid from v$session; ---查询用户会话
alter system kill session 'serial#, sid ';---删除相关用户会话

建议以后台登陆删除用户会话
1、查询oracle的连接数
select count(*) from v$session;
2、查询oracle的并发连接数
select count(*) from v$session where status='active';
3、查看不同用户的连接数
select username,count(username) from v$session where username is not null group by username;
4、查看所有用户：
select * from all_users;
5、查看用户或角色系统权限(直接赋值给用户或角色的系统权限)：
select * from dba_sys_privs;
select * from user_sys_privs;
6、查看角色(只能查看登陆用户拥有的角色)所包含的权限
select * from role_sys_privs;
7、查看用户对象权限：
select * from dba_tab_privs;
select * from all_tab_privs;
select * from user_tab_privs;
8、查看所有角色：
select * from dba_roles;
9、查看用户或角色所拥有的角色：
select * from dba_role_privs;
select * from user_role_privs;
10、查看哪些用户有sysdba或sysoper系统权限(查询时需要相应权限)
select * from v$pwfile_users;

修改数据库允许的最大连接数：
alter system set processes = 300 scope = spfile;

查看游标数量
select * from v$open_cursor where user_name=''

查询数据库允许的最大连接数：
select value from v$parameter where name = 'processes';
或者：show parameter processes;

查询数据库允许的最大游标数：
select value from v$parameter where name = 'open_cursors'

查看oracle版本
select banner from sys.v_$version;

按降序显示用户"system"为每个会话打开的游标数
select o.sid, osuser, machine, count(*) num_curs from v$open_cursor o, v$session s where user_name = 'system' and o.sid=s.sid group by o.sid, osuser, machine order by num_curs desc;

以上转载自 https://www.cnblogs.com/caogang/p/4546072.html

保留两位小数以gb为单位显示数据文件大小
select round(sum(bytes)/1024/1024/1024,2)||'gb' from dba_data_files;

以mb为单位表空间分组显示数据文件大小
select tablespace_name,sum(bytes)/1024/1024 as mb from dba_data_files
group by tablespace_name;

显示数据库中临时文件的大小
select sum(bytes)/1024/1024/1024 as gb from dba_temp_files;

显示数据库中日志文件的大小
select sum(bytes)/1024/1024/1024 as gb from v$log;

显示数据库中数据文件、临时文件、日志文件的总大小
select sum(gb) as gb
from(
select sum(bytes)/1024/1024/1024 as gb
from dba_data_files
union all
select sum(bytes)/1024/1024/1024
from dba_temp_files
union all
select sum(bytes)/1024/1024/1024
from v$log
);

select round(sum(bytes) / 1024 / 1024, 2) "used(m)"
from dba_segments
group by owner, tablespace_name
order by sum(bytes) desc;

--1、查看所有表空间及表空间大小：
select tablespace_name ,sum(bytes) / 1024 / 1024 as mb　from dba_data_files group by tablespace_name;

--2、查看所有表空间对应的数据文件：
select tablespace_name,file_name from dba_data_files;

--3、修改数据文件大小：
alter database datafile 'h:\oracle\product\10.1.0\oradata\oracle\users01.dbf' resize 10240m;

select file_name,bytes/1024/1024 mb from dba_data_files;