StepbyStep Guide: How to Modify the System Time in Oracle（修改oracle系统时间）

Introduction

The system time in Oracle plays a crucial role in numerous operations, such as scheduling jobs, managing transactions, and troubleshooting performance issues. Occasionally, you may need to modify the system time to fix errors, test the behavior of time-dependent features, or accommodate daylight saving changes. In this step-by-step guide, we’ll explore the different methods to change the system time in Oracle, including the recommended and unsupported techniques.

Method 1: Using the SET_TIME Procedure

The SET_TIME procedure is a preferred way to adjust the system time in Oracle because it updates the kernel time, the database time, and the session time simultaneously, ensuring coherence across all levels. Here’s how to use the SET_TIME procedure:

1. Connect to the Oracle database as a privileged user, such as SYS or SYSTEM.

2. Run the SET_TIME procedure with the desired time value in the format ‘YYYY-MM-DD HH24:MI:SS’:

“`SQL

EXEC DBMS_SCHEDULER.SET_TIME(‘YYYY-MM-DD HH24:MI:SS’);

3. Check the system time by executing the following query:

```SQL
SELECT SYSTIMESTAMP FROM DUAL;

Method 2: Using the ALTER SYSTEM Command

The ALTER SYSTEM command is an alternative way to modify the system time in Oracle, but it updates only the database time and requires rebooting the instance to synchronize the kernel time. Therefore, it’s preferable to use the SET_TIME procedure unless the instance is already scheduled for maintenance. Here’s how to use the ALTER SYSTEM command:

1. Connect to the Oracle database as a privileged user, such as SYS or SYSTEM.

2. Run the ALTER SYSTEM command with the desired time value in the format ‘YYYY-MM-DD HH24:MI:SS’:

“`SQL

ALTER SYSTEM SET TIME_ZONE = ‘YYYY-MM-DD HH24:MI:SS’;

3. Restart the instance to apply the changes.

Method 3: Using the NTP Service
The Network Time Protocol (NTP) is an Internet protocol that synchronizes the system time across multiple devices with high accuracy and reliability. Oracle recommends using the NTP service to maintain the correct time on your systems and configure it to synchronize at regular intervals. Here's how to enable and configure the NTP service on Linux:
1. Install the ntp package if it's not already installed:

```bash
sudo yum install ntp

2. Start the NTP service and set it to start automatically on boot:

“`bash

sudo systemctl start ntpd.service

sudo systemctl enable ntpd.service

3. Open the /etc/ntp.conf file in a text editor and add the NTP servers you want to use:

```bash
server ntp1.example.com
server ntp2.example.com

4. Save and close the file and restart the NTP service to apply the changes:

“`bash

sudo systemctl restart ntpd.service

Conclusion
In this guide, we've reviewed the different techniques to modify the system time in Oracle, including the recommended and unsupported ways. While the SET_TIME procedure is the most comprehensive and reliable method to adjust the time, it may not always be available, especially in older versions of Oracle. Therefore, the ALTER SYSTEM command and the NTP service can be used as alternatives to meet your specific needs. Remember to test any change to the system time carefully and consider the impact on other applications and users.