有時我們資料庫需要進入限制模式，比如維護時。那麼，如何才能讓系統進入限制模式呢 ？
1.開啟資料庫直接進入限制模式
startup open restricted;
2.修改開啟的資料庫進行限制模式
alter system enable restricted session;
看監聽服務的狀態：
lsnrctl> status
connecting to (description=(address=(protocol=tcp)(host=dbserver)(port=1521)))
status of the listener
alias listener
version tnslsnr for linux: version 12.1.0.2.0 - production
start date 01-dec-2019 09:50:50
uptime 0 days 5 hr. 16 min. 35 sec
trace level off
security on: local os authentication
snmp off
listening endpoints summary...
(description=(address=(protocol=tcp)(host=dbserver.sztech.com)(port=1521)))
(description=(address=(protocol=ipc)(key=extproc1521)))
services summary...
service "cdb1" has 1 instance(s).
instance "cdb1", status restricted, has 1 handler(s) for this service...
service "cdb1xdb" has 1 instance(s).
instance "cdb1", status restricted, has 1 handler(s) for this service...
service "emrep" has 1 instance(s).
instance "emrep", status ready, has 1 handler(s) for this service...
service "emrepxdb" has 1 instance(s).
instance "emrep", status ready, has 1 handler(s) for this service...
service "orcl" has 1 instance(s).
instance "orcl", status restricted, has 1 handler(s) for this service...
service "orclxdb" has 1 instance(s).
instance "orcl", status restricted, has 1 handler(s) for this service...
service "pdb1_1" has 1 instance(s).
instance "cdb1", status restricted, has 1 handler(s) for this service...
service "pdb2" has 1 instance(s).
instance "cdb1", status restricted, has 1 handler(s) for this service...
the command completed successfully
lsnrctl>
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