Active Data Guard 备用数据库可以运行查询操作或只读应用程序吗？

可以的，官方文档有说明

下面的功能是允许在read-only的数据库上执行的：
• Issue SELECT statements, including queries that require multiple sorts that leverage TEMP segments
• Use ALTER SESSION and ALTER SYSTEM statements
• Use SET ROLE
• Call stored procedures
• Use database links (dblinks) to write to remote databases
• Use stored procedures to call remote procedures via dblinks
• Use SET TRANSACTION READ ONLY for transaction level read consistency
• Issue complex queries (such as grouping SET queries and WITH CLAUSE queries)

下面的功能是不允许在read-only的数据库上执行的：
• Any DMLs (excluding simple SELECT statements) or DDLs
• Query accessing local sequences
• DMLs to local temporary tables

可以的，官方文档有说明

下面的功能是允许在read-only的数据库上执行的：
• Issue SELECT statements, including queries that require multiple sorts that leverage TEMP segments
• Use ALTER SESSION and ALTER SYSTEM statements
• Use SET ROLE
• Call stored procedures
• Use database links (dblinks) to write to remote databases
• Use stored procedures to call remote procedures via dblinks
• Use SET TRANSACTION READ ONLY for transaction level read consistency
• Issue complex queries (such as grouping SET queries and WITH CLAUSE queries)

下面的功能是不允许在read-only的数据库上执行的：
• Any DMLs (excluding simple SELECT statements) or DDLs
• Query accessing local sequences
• DMLs to local temporary tables