Types of Standby Databases in Oracle Data Guard
Oracle Data Guard uses standby databases to maintain a real-time or near-real-time replica of your primary database. These standby databases help ensure disaster recovery (DR), high availability (HA), and even testing environments — depending on the type.
There are three main types:
- Physical Standby
- Logical Standby
- Snapshot Standby
Each is designed for a specific scenario, and choosing the right one depends on your goals: HA, reporting, or testing.
1️⃣ Physical Standby
Exact block-for-block copy of the primary
A Physical Standby is a live mirror of the primary database. It applies redo logs directly using Redo Apply, ensuring the standby is always in sync and ready for failover.
| Category | Details |
|---|---|
| ✅ Best For | – High availability (HA) – Disaster recovery (DR) – Read-only queries (with ADG) |
| ⚙️ Key Strengths | – Fast failover/switchover – Block-level accuracy – Minimal performance overhead |
| 🚫 Limitations | – No read-write – Same structure as primary – Not suitable for development/test tasks |
| 💡 Example | Think of it as a real-time mirror — if the main system fails, the standby takes over instantly. |
2️⃣ Logical Standby
Same data, but flexible structure using SQL Apply
A Logical Standby replays redo as SQL statements. This gives it flexibility to support different indexes, views, and even additional columns, making it ideal for reporting or upgrades.
| Category | Details |
|---|---|
| ✅ Best For | – Reporting & analytics – Schema changes – Rolling upgrades |
| ⚙️ Key Strengths | – Read-write access – Custom indexes/views – Supports limited structural changes |
| 🚫 Limitations | – Slower sync – Doesn’t support all object types – Less preferred for critical HA setups |
| 💡 Example | Acts like a SQL interpreter — instead of raw data blocks, it runs SQL to stay updated. |
3️⃣ Snapshot Standby
Temporary read-write version of a Physical Standby
A Snapshot Standby is a paused version of a Physical Standby. Redo logs are held back while the database becomes fully writable, making it ideal for testing or QA with near-live data.
Once done, you discard the changes and convert it back to a synced Physical Standby.
| Category | Details |
|---|---|
| ✅ Best For | – QA & UAT – Patch testing – Safe dev on real data |
| ⚙️ Key Strengths | – Writable with real production-like data – Easy to revert back to sync mode |
| 🚫 Limitations | – No sync during snapshot mode – No failover until converted – Not suitable for DR/HA |
| 💡 Example | Think of it as a testing clone — you test all you want, then revert it back to standby mode safely. |
Quick Comparison Table
| Type | Sync Method | Read-Only | Read-Write | Auto Failover | Best Use Case |
|---|---|---|---|---|---|
| Physical Standby | Redo Apply | ✅ (with ADG) | ❌ | ✅ | High availability & DR |
| Logical Standby | SQL Apply | ✅ | ✅ | ⚠️ Limited | Reporting & flexible schema |
| Snapshot Standby | Redo paused | ❌ | ✅ | ❌ | Safe dev/testing on live data |
In One Line:
Physical = Mirror, Logical = Translator, Snapshot = Test Lab.