1 说明

In previous releases, all database instances—unless they accessed a standby database—were read/write. Starting in Oracle Database 12c Release 2 (12.2), read-only and read/write instances can co-exist within a single database. This configuration is useful for parallel SQL statements that both query and modify data, because both read/write and read-only instances can query, while the read/write instances modify.http://www.cndba.cn/Expect-le/article/2542http://www.cndba.cn/Expect-le/article/2542

在12.2中，在同一个数据库中可以同时打开读写和只读两个实例。

只读实例特点：

1. Can only open a database that has already been opened by a read/write instancehttp://www.cndba.cn/Expect-le/article/2542

2. Disable many background processes, including the checkpoint and archiver processes, which are not necessaryhttp://www.cndba.cn/Expect-le/article/2542

3. Can mount a disabled redo thread or a thread without any online redo log

2 指定实例为只读

需要配置初始化参数INSTANCE_MODE为READ_ONLY，默认值为READ_WRITE。

更多详细信息，参考官方文档：

https://docs.oracle.com/en/database/oracle/oracle-database/12.2/cncpt/oracle-database-instance.html#GUID-8BB8140D-63ED-454E-AAC3-1964F80D102Dhttp://www.cndba.cn/Expect-le/article/2542

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read_only read_write