Difference between physical and logical standby?
In the world of Oracle Data Guard, choosing between a Physical and Logical standby is like choosing between a Mirror and a Copyist. One mimics every single move exactly as it happens, while the other interprets the instructions and recreates the work in its own way.
While Physical standbys are the industry standard for Disaster Recovery, Logical standbys offer unique flexibility for specific use cases.
A Physical Standby is a bit-for-bit, block-for-block identical copy of the primary database. It uses Redo Apply technology.
How it works: It receives redo logs from the primary and applies them directly to the data blocks. It doesn't care about SQL; it just ensures the physical files match.
The State: Traditionally, it is in a MOUNTED state (not open). With Active Data Guard, it can be open in READ ONLY mode while still applying changes.
Best For: Disaster Recovery, data protection, and offloading backups.
A Logical Standby contains the same data as the primary, but its physical structure (data files, indexes, organization) can be completely different. It uses SQL Apply technology.
How it works: It receives redo logs, but then it performs "Log Mining" to translate that redo back into SQL statements (INSERT, UPDATE, DELETE). It then executes those SQL statements on the standby.
The State: It is always open in READ-WRITE mode.
Best For: Reporting with custom indexes, rolling upgrades, and specific data transformations.
| Feature | Physical Standby | Logical Standby |
| Apply Mechanism | Redo Apply (Block-level) | SQL Apply (Row-level) |
| Data Consistency | Identical (Bit-for-bit) | Logically Equivalent |
| Complexity | Simple to manage | Higher (Requires Log Mining) |
| Supported Data Types | All Oracle data types | Limited (Some complex types unsupported) |
| Customization | No (Cannot add indexes/tables) | Yes (Can add local indexes/tables) |
| Performance | Extremely fast | Slower (SQL execution overhead) |
Pros: Fastest performance, easiest to set up, supports all features (including RAC and specialized data types), and provides the best protection against data corruption.
Cons: You generally cannot modify the schema or add unique indexes just for reporting.
Pros: You can create additional indexes or materialized views that exist only on the standby to speed up specific reports. It is also great for Rolling Upgrades, allowing you to upgrade the standby to a new Oracle version while the primary stays on the old one.
Cons: It is much harder to maintain. If a SQL statement fails to apply (e.g., due to an unsupported data type), the standby "lags" or stops until you fix it manually.
Choose Physical if your main goal is Disaster Recovery (DR). It is the most reliable way to ensure you don't lose data. Most modern shops use Physical Standby with the Active Data Guard option to get the best of both worlds (DR + Reporting).
Choose Logical only if you have a very specific requirement, like needing different indexes for reporting that would slow down the primary, or if you are performing a complex version upgrade with near-zero downtime.
Peer Tip: In recent years, Active Data Guard (Physical) has become so powerful that Logical Standbys have become quite rare. Unless you have a specific "Rolling Upgrade" or "Custom Indexing" requirement that ADG can't handle, stick with a Physical Standby. It’s significantly less "fussy."