System Global Area

What is SGA in Oracle?

When Oracle says it’s “allocating memory” at startup, it’s setting up the System Global Area (SGA) — a shared memory region used by the Oracle instance to process and manage data efficiently.

It’s like the central memory workspace for the database.

🧩 SGA = Shared Memory for the Whole Instance

The SGA is accessed by all background and server processes. It holds:

  • Parsed SQL & PL/SQL
  • Frequently accessed data blocks
  • Redo log entries
  • Control structures

Without the SGA, Oracle couldn’t cache data or execute queries efficiently.

🛠 When the instance starts (STARTUP), Oracle allocates the SGA based on parameters like SGA_TARGET or MEMORY_TARGET.

🔍 Core Components of SGA

🔹 1. Shared Pool

  • Stores parsed SQL & PL/SQL code
  • Contains data dictionary & system metadata
  • Crucial for SQL parsing and execution

🔎 Related Views: V$SHARED_POOL_ADVICE, V$SQLAREA

🔹 2. Database Buffer Cache

  • Holds copies of data blocks read from disk
  • Reduces physical I/O
  • Shared by all sessions

🧠 Hot blocks stay in cache longer — tuning this improves query speed.

🔹 3. Redo Log Buffer

  • Temporary memory area for redo entries (DML, DDL)
  • Flushed to redo logs by LGWR
  • Helps with crash recovery and consistency

🔁 Writes are short-lived — size carefully to avoid bottlenecks.

🔹 4. Fixed SGA

  • Small, untunable memory region
  • Stores internal info like process state and system stats
  • Allocated during instance startup

⚙️ Optional (Advanced) SGA Components

These components are not always enabled — you configure them based on features used:

ComponentUse Case
Large PoolRMAN, Shared Server, Parallel Query, XA
Java PoolRunning Java inside Oracle DB
Streams PoolOracle Streams, GoldenGate, Data Pump
In-Memory AreaIn-Memory Column Store for analytics
Memoptimize PoolFast key-value lookups
Shared I/O PoolSecureFile LOB I/O optimization
Flashback BufferFlashback Database recovery
Database Smart Flash CacheExtends Buffer Cache to flash disk (Linux/Solaris)
Vector PoolVector indexes (AI/ML queries, IVF/HNSW)
True CacheHigh-speed memory cache for hot tables

🔍 Useful SGA Queries

✅ Show Overall SGA Usage:

SHOW SGA;

✅ Check Dynamic SGA Components:

SELECT * FROM v$sga_dynamic_components;

✅ List All SGA Parameters:

SET LINESIZE 200
SET PAGESIZE 100

COLUMN name FORMAT A40
COLUMN value FORMAT A30

SELECT name, value 
FROM v$parameter 
WHERE name LIKE 'sga_%'
ORDER BY name;

✅ Query : View SGA Overview Information (v$sgainfo)

SET LINESIZE 200
SET PAGESIZE 100

COLUMN NAME FORMAT A50
COLUMN BYTES FORMAT 999,999,999,999
COLUMN RESIZEABLE FORMAT A15

SELECT * 
FROM v$sgainfo;

✅ Query : View Detailed SGA Statistics (v$sgastat)

SET LINESIZE 200
SET PAGESIZE 200

COLUMN POOL FORMAT A20
COLUMN NAME FORMAT A50
COLUMN BYTES FORMAT 999,999,999,999

SELECT * 
FROM v$sgastat 
ORDER BY pool, name;

🛠️ DBA Tip of the Day

“When troubleshooting performance — start with the SGA.”

Most slow queries, memory issues, or I/O bottlenecks stem from:

  • An undersized Buffer Cache
  • A fragmented Shared Pool
  • Redo log buffer contention

📊 Use views like V$SGAINFO, V$SGASTAT, and V$MEMORY_TARGET_ADVICE to monitor memory and tune the instance proactively.